Semiconductor device and manufacturing method thereof

ABSTRACT

An object is to reduce a capacitance value of parasitic capacitance without decreasing driving capability of a transistor in a semiconductor device such as an active matrix display device. Further, another object is to provide a semiconductor device in which the capacitance value of the parasitic capacitance was reduced, at low cost. An insulating layer other than a gate insulating layer is provided between a wiring which is formed of the same material layer as a gate electrode of the transistor and a wiring which is formed of the same material layer as a source electrode or a drain electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and amanufacturing method thereof.

2. Description of the Related Art

A so-called flat panel display (FPD) typified by a liquid crystaldisplay device has characteristics of being thin and low powerconsumption. Therefore, flat panel displays are widely used in variousfields. Among them, since an active matrix liquid crystal display devicehaving a thin film transistor (TFT) in each pixel has high displayperformance, the market size is remarkably being expanded.

A plurality of scanning lines and signal lines is formed over an activematrix substrate used for an active matrix display device and thesewirings intersect with each other with an insulating layer interposedtherebetween. Thin film transistors are provided close to anintersection portion of the scanning line and the signal line and eachpixel is switched (e.g., see Patent Document 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. H04-220627

Here, electrostatic capacitance (also called “parasitic capacitance”) isformed in the intersection portion of the scanning line and the signalline because of its structure. Since parasitic capacitance causes signaldelay or the like and makes display quality decreased, a capacitancevalue thereof is preferably small.

As a method for reducing parasitic capacitance which is generated in theintersection portion of the scanning line and the signal line, forexample, a method for forming an insulating film thick which covers thescanning line is given; however, in a bottom-gate transistor, a gateinsulating layer is formed between the scanning line and the signalline, whereby, driving capability of a transistor is decreased in thecase where the gate insulating layer is simply formed thick.

SUMMARY OF THE INVENTION

In view of the foregoing problems, in a semiconductor device such as anactive matrix display device, an object is to reduce the capacitancevalue of the parasitic capacitance without decreasing driving capabilityof a transistor. Further, another object is to provide a semiconductordevice in which the capacitance value of the parasitic capacitance wasreduced at low cost.

In the present invention disclosed, an insulating layer other than agate insulating layer is provided between a wiring which is formed ofthe same material layer as a gate electrode of the transistor and awiring which is formed of the same material layer as a source electrodeor a drain electrode.

An embodiment of the present invention disclosed in this specificationis a method for manufacturing a semiconductor device including the stepsof: forming a first conductive layer over a substrate; selectivelyforming a resist mask with plural thicknesses over the first conductivelayer; etching the first conductive layer using the resist mask andforming a gate electrode and a first wiring; making the resist maskrecede to remove a resist mask over the gate electrode and leaving partof the resist mask over the first wiring; forming a gate insulatinglayer so as to cover the gate electrode, the first wiring, and theresist mask which is left; forming a second conductive layer over thegate insulating layer; selectively etching the second conductive layerto form a source and drain electrodes and forming a second wiringoverlapping the first wiring in a region overlapped with the resist maskwhich is left; and forming a semiconductor layer which is in contactwith the source and drain electrodes in a region overlapped with thegate electrode.

In the above description, an oxide semiconductor layer containingindium, gallium, and zinc may be formed as the semiconductor layer.

In the above description, the first wiring is preferably formed so thatthe width of the first wiring in a region overlapped with the resistmask which is left is smaller than the width of the first wiring in theother regions. Further, the second wiring is preferably formed so thatthe width of the second wiring in a region overlapped with the resistmask which is left is smaller than the width of the second wiring in theother regions.

In addition, the first wiring is preferably formed so that the thicknessof the first wiring in the region overlapped with the resist mask whichis left is larger than the thickness of the first wiring in the otherregions. Further, the second wiring is preferably formed so that thethickness of the second wiring in the region overlapped with the resistmask which is left is larger than the thickness of the second wiring inthe other regions. For example, another conductive layer is preferablyformed over the second wiring. Note that the first wiring and the secondwiring may have either a single-layer structure or a stacked-layerstructure.

Note that in this specification, a semiconductor device refers to anydevice which can function by utilizing semiconductor characteristics; adisplay device, a semiconductor circuit, an electronic appliance are allincluded in the category of the semiconductor device.

According to one embodiment of the present invention disclosed, a resistmask used in forming the first wiring is partly left, whereby acapacitance value of parasitic capacitance formed by the first wiringand the second wiring is reduced. Thus, a semiconductor device in whichthe capacitance value of the parasitic capacitance is reduced can beprovided while suppressing increase in the number of manufacturingsteps.

Further, in the case where the width of the first wiring or the secondwiring is small in a region where these wirings are overlapped with eachother, the capacitance value of the parasitic capacitance can be furtherreduced.

On the other hand, in the case where the width of the wiring is locallysmall as described above, wiring resistance in the region is increased.In order to solve this problem, the thickness of the wiring in theregion is preferably increased. In the case where a thickness of awiring is increased, an increase in local wiring resistance can besuppressed and characteristics of a semiconductor device can bemaintained. Note that in the present invention disclosed, a thickness ofa wiring can be increased while the number of steps can be suppressed.

Through the above steps, according to one embodiment of the presentinvention disclosed, a high-performance semiconductor device in which acapacitance value of parasitic capacitance is reduced can be provided atlow cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 1.

FIGS. 2A to 2C are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 1.

FIGS. 3A to 3D are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 2.

FIGS. 4A to 4D are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 2.

FIGS. 5A to 5E are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 3.

FIGS. 6A to 6C are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 4.

FIGS. 7A to 7C are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 4.

FIG. 8 is a plan view of a semiconductor device of Embodiment 4.

FIGS. 9A to 9C are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 5.

FIG. 10 is a plan view of a semiconductor device of Embodiment 5.

FIGS. 11A-1, 11A-2 and 11B are views illustrating a semiconductor deviceof Embodiment 6.

FIG. 12 is a view illustrating a semiconductor device of Embodiment 6.

FIG. 13 is a view illustrating a semiconductor device of Embodiment 7.

FIGS. 14A to 14C are views illustrating a semiconductor device ofEmbodiment 8.

FIGS. 15A and 15B are views illustrating a semiconductor device ofEmbodiment 8.

FIGS. 16A and 16B are views illustrating examples of usage patterns ofelectronic paper.

FIG. 17 is an external view illustrating an example of an electronicbook reader.

FIG. 18A is an external view of an example of a television device andFIG. 18B is an external view of an example of a digital photo frame.

FIGS. 19A and 19B are external views illustrating examples of anamusement machine.

FIGS. 20A and 20B are external views illustrating examples of a cellularphone.

FIGS. 21A to 21D are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 11.

FIGS. 22A to 22D are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 12.

FIGS. 23A to 23D are cross-sectional views illustrating a method formanufacturing a semiconductor device of Embodiment 13.

FIGS. 24A and 24B are cross-sectional views showing structures oftransistors of Example 1.

FIGS. 25A and 25B are graphs showing electric characteristics oftransistors of Example 1.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments are described in detail with reference to the accompanyingdrawings. Note that the present invention is not limited to thedescription in the embodiments below, and it is apparent to thoseskilled in the art that modes and details of the present invention canbe changed in various ways without departing from its spirit. Inaddition, structures according to different embodiments can beimplemented in combination as appropriate. Note that in the structuresof the invention described below, the same portions or portions havingsimilar functions are denoted by the same reference numerals, andrepetitive description thereof is omitted.

Embodiment 1

In this embodiment, an example of a method for manufacturing asemiconductor device is described with reference to drawings.

First, a conductive layer 102 is formed over a substrate 100 and resistmasks 104 and 106 are selectively formed over the conductive layer 102(see FIG. 1A). Note that in this embodiment, the resist mask 106 isformed thicker than the resist mask 104.

Any substrate can be used for the substrate 100 as long as it is asubstrate having an insulating surface, for example, a glass substrate.It is preferable that the glass substrate be a non-alkali glasssubstrate. As a material of the non-alkali glass substrate, a glassmaterial such as aluminosilicate glass, aluminoborosilicate glass,barium borosilicate glass, or the like is used, for example. Besides, asthe substrate 100, an insulating substrate formed of an insulator suchas a ceramic substrate, a quartz substrate, or a sapphire substrate, asemiconductor substrate formed of a semiconductor material such assilicon, over which an insulating material is covered, a conductivesubstrate formed of a conductive material such as metal or stainlesssteel, over which an insulating material is covered can be used. Aplastic substrate can also be used as long as it can withstand thermaltreatment in a manufacturing step.

The conductive layer 102 is preferably formed of a conductive materialsuch as aluminum (Al), copper (Cu), molybdenum (Mo), tungsten (W),titanium (Ti). As a formation method, a sputtering method, a vacuumevaporation, a CVD method, and the like are given. In the case of usingaluminum (or copper) for the conductive layer 102, since aluminum itself(or copper itself) has disadvantages such as low heat resistance and atendency to be corroded, it is preferably formed in combination with aconductive material having heat resistance.

As the conductive material having heat resistance, it is possible to usemetal containing an element selected from titanium (Ti), tantalum (Ta),tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), andscandium (Sc), an alloy containing any of these elements as itscomponent, an alloy containing a combination of any of these elements, anitride containing any of these elements as its component, or the like.The conductive material having heat resistance and aluminum (or copper)may be stacked, whereby the conductive layer 102 may be formed.

The resist masks 104 and 106 can be formed using a multi-tone mask.Here, the multi-tone mask is a mask capable of light exposure withmulti-level light intensity. With the use of a multi-tone mask, one-timeexposure and development process allow a resist mask with pluralthicknesses (typically, two kinds of thicknesses) to be formed. By useof the multi-tone mask, the number of steps can be suppressed.

For example, in order to form a resist mask with two kinds ofthicknesses, light exposure is preferably performed using a multi-tonemask which is irradiated with three levels of light intensity to providean exposed region, a half-exposed region, and an unexposed region.

As a multi-tone mask, a gray-tone mask and a half-tone mask are given. Agray-tone mask can have a structure having a light blocking portionformed using a light blocking layer, a slit portion provided by apredetermined pattern of the light blocking film, and a transmittingportion where these are not provided, over a substrate having alight-transmitting property. A half-tone mask can have a structurehaving a light blocking portion formed using a light blocking layer, asemi-transmitting portion formed using a semi-transmissive film, and atransmitting portion where these are not provided, over a substratehaving a light-transmitting property.

The light blocking film for forming the light blocking portion and theslit portion may be formed using a metal material, and for example, thelight blocking film is preferably formed using chromium, chromium oxide,or the like.

In addition, the slit portion has slits (including dots, meshes, or thelike) which are provided in size which is less than or equal to thediffraction limit (also referred to as a resolution limit) of light usedfor exposure. Thus, light transmittance is controlled. Note that theslit portion 143 may have slits with either regular or irregularintervals.

The semi-light-transmitting portion can be formed using MoSiN, MoSi,MoSiO, MoSiON, CrSi, or the like having a light-transmitting property.

By light exposure using such a multi-tone mask and development, theresist masks 104 and 106 having different thicknesses can be formed.

Note that a method for manufacturing the resist masks 104 and 106 arenot limited to the above method. The above resist masks may be formed bya method by which films having different thicknesses can be selectivelyformed such as an ink-jet method.

Next, the conductive layer 102 is etched using the above resist masks104 and 106, so that a gate electrode 108 and a first wiring 110 areformed (see FIG. 1B).

As the above etching treatment, dry etching may be used, or wet etchingmay be used. In order to improve coverage of a gate insulating layer orthe like which is formed later and prevent disconnection, the etching ispreferably performed so that end portions of the gate electrode 108 andthe first wiring 110 are tapered. For example, the end portions arepreferably tapered at a taper angle 20° or more and less than 90°. Here,the “taper angle” refers to an angle formed by a side surface of a layerwhich is tapered to a bottom surface thereof when the layer having atapered shape is observed from a cross-sectional direction.

Next, the resist masks 104 and 106 are made to recede to expose asurface of the gate electrode 108, whereby a resist mask 112 is formedover the first wiring 110 (see FIG. 1C). As a method for making theresist masks 104 and 106 to recede, for example, ashing treatment usingoxygen plasma can be given; however, the present invention disclosed isnot interpreted as being limited to the method.

Next, a gate insulating layer 114 is formed so as to cover the gateelectrode 108, the first wiring 110, and the resist mask 112 (see FIG.1D). The gate insulating layer 114 can be formed using a material suchas silicon oxide, silicon oxynitride, silicon nitride, silicon nitrideoxide, aluminum oxide, or tantalum oxide. The insulating layer 114 mayalso be formed by stacking films formed of these materials. These filmsare preferably formed to a thickness of greater than or equal to 5 nmand less than or equal to 250 nm by a sputtering method or the like. Forexample, as the gate insulating layer 114, a silicon oxide film can beformed to a thickness of 100 nm by a sputtering method.

Alternatively, the gate insulating layer 114 with a stacked-layerstructure may be formed by combination of a sputtering method and a CVDmethod (a plasma CVD method or the like). For example, a lower layer ofthe gate insulating layer 114 (a region in contact with the gateelectrode 108) is formed by a plasma CVD method and an upper layer ofthe gate insulating layer 114 is formed by a sputtering method. Since afilm with favorable step coverage is easily formed by a plasma CVDmethod, it is suitable for a method for forming a film just above thegate electrode 108. In the case of using a sputtering method, since itis easy to reduce hydrogen concentration in the film as compared to thecase of using a plasma CVD method, by providing a film by a sputteringmethod in a region in contact with a semiconductor layer, the hydrogenin the gate insulating layer 114 can be prevented from being diffusedinto the semiconductor layer. In particular, in the case where asemiconductor layer is formed using an oxide semiconductor material,since it is considered that hydrogen has a great influence oncharacteristics, it is effective to employ such a structure.

Note that in this specification, oxynitride refers to a substance thatcontains more oxygen (number of atoms) than nitrogen. For example,silicon oxynitride is a substance containing oxygen, nitrogen, silicon,and hydrogen in ranges of 50 atomic % to 70 atomic %, 0.5 atomic % to 15atomic %, 25 atomic % to 35 atomic %, and 0.1 atomic % to 10 atomic %,respectively. Further, nitride oxide refers to a substance that containsmore nitrogen (number of atoms) than oxygen. For example, siliconnitride oxide is a substance containing oxygen, nitrogen, silicon, andhydrogen in ranges of 5 atomic % to 30 atomic %, 20 atomic % to 55atomic %, 25 atomic % to 35 atomic %, and 10 atomic % to 25 atomic %,respectively. Note that the above ranges are ranges for cases wheremeasurement is performed using Rutherford backscattering spectrometry(RBS) and hydrogen forward scattering spectrometry (HFS). Moreover, thetotal for the content ratio of the constituent elements does not exceed100 atomic %.

Next, a conductive layer 116 is formed over a gate insulating layer 114(see FIG. 2A). The conductive layer 116 can be formed using a materialand by a method which are similar to those of the conductive layer 102.For example, the conductive layer 116 can be formed to have asingle-layer structure of a molybdenum film or a titanium film.Alternatively, the conductive layer 116 may be formed to have astacked-layer structure and can have a stacked-layer structure of analuminum film and a titanium film, for example. A three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order may be employed. A three-layer structure in whicha molybdenum film, an aluminum film, and a molybdenum film are stackedin this order may be employed. Further, an aluminum film containingneodymium (an Al—Nd film) may be used as the aluminum film used forthese stacked-layer structures. Further alternatively, the conductivelayer 116 may have a single-layer structure of an aluminum filmcontaining silicon.

Next, the conductive layer 116 is selectively etched to form a sourceelectrode 118, a drain electrode 120, and a second wiring 122 (see FIG.2B).

Note that the source electrode 118 may function as the drain electrodeand the drain electrode 120 may function as the source electrodedepending on a method for driving a transistor. Therefore, denominationsof source and drain can be switched depending on the function or thecondition. In addition, these denominations are ones of convenience andare not ones which determine their functions.

Although not described in this embodiment, after the above steps, thegate insulating layer 114, the source electrode 118, and the drainelectrode 120 may be subjected to surface treatment. As the surfacetreatment, plasma treatment using an inactive gas and/or a reactive gasor the like can be applied.

Plasma treatment can be, for example, performed in a plasma state byintroducing an inert gas such as an argon (Ar) gas into a chamber in avacuum state and applying a bias voltage to an object. When an Ar gas isintroduced into a chamber, electrons and Ar cations are present inplasma, and the Ar cations are accelerated in a cathode direction. Theaccelerated Ar cations collide with surfaces of the gate insulatinglayer 114, the source electrode 118, and the drain electrode 120 whichare formed over the substrate 100, whereby the surfaces are etched bysputtering and the surfaces of the gate insulating layer 114, the sourceelectrode 118, and the drain electrode 120 can be modified. Note thatsuch plasma treatment may also be called “reverse sputtering” treatment.

When plasma treatment is performed by application of bias voltage to thesubstrate 100 side, the surfaces of the gate insulating layer 114, thesource electrode 118, and the drain electrode 120 can be effectivelyetched by sputtering. In addition, when projections and depressions areformed on the surface of the gate insulating layer 114, the projectionsof the gate insulating layer 114 are preferentially etched by sputteringby plasma treatment, so that the planarity of the surface of the gateinsulating layer 114 can be improved.

As the above plasma treatment, a helium gas can be used in addition toan argon gas. Alternatively, an atmosphere in which oxygen, hydrogen,nitrogen, or the like is added to an argon gas or a helium gas may beused. Further alternatively, an atmosphere in which Cl₂, CF₄, or thelike is added to an argon gas or a helium gas may be used.

Next, after a semiconductor layer is formed so as to cover the gateinsulating layer 114, the source electrode 118, and the drain electrode120, the semiconductor layer is selectively etched, so that anisland-shape semiconductor layer 124 is formed in which at least partthereof is in contact with the source electrode 118 and the drainelectrode 120 (see FIG. 2C). There is no particular limitation on amaterial used for the island-shape semiconductor layer 124. Theisland-shape semiconductor layer 124 can be formed using, for example, asilicon-based semiconductor material such as single crystal silicon,polycrystalline silicon or amorphous silicon, a germanium-basedsemiconductor material, or the like. Alternatively, a compoundsemiconductor material such as silicon germanium, silicon carbide,gallium arsenide, or indium phosphide may be used. In particular, whenan oxide semiconductor material (a metal oxide semiconductor material)is used, a semiconductor device with excellent characteristics can beprovided. In this embodiment, the case where an oxide semiconductormaterial is used as the island-shape semiconductor layer 124 isdescribed.

Note that as an example of the above oxide semiconductor material, onerepresented by InMO₃ (ZnO)_(m) (m>0) is given. Here, M denotes one ormore of metal elements selected from gallium (Ga), iron (Fe), nickel(Ni), manganese (Mn), and cobalt (Co). For example, when Ga is selectedas M, the case where the above metal element other than Ga, such as Gaand Ni or Ga and Fe, is included in addition to the case where only Gais selected. Moreover, in the above oxide semiconductor, in some cases,a transition metal element such as Fe or Ni or an oxide of thetransition metal is contained as an impurity element in addition to ametal element contained as M. Needless to say, the oxide semiconductormaterial is not limited to the above materials and a variety of oxidesemiconductor materials such as zinc oxide or indium oxide can be used.

An insulating impurity may be contained in the oxide semiconductor. Asthe impurity, insulating oxide typified by silicon oxide, germaniumoxide, aluminum oxide, or the like; insulating nitride typified bysilicon nitride, aluminum nitride, or the like; or insulating oxynitridesuch as silicon oxynitride or aluminum oxynitride is applied.

The insulating oxide or the insulating nitride is added to the oxidesemiconductor at a concentration at which electrical conductivity of theoxide semiconductor does not deteriorate.

Insulating impurity is contained in the oxide semiconductor, wherebycrystallization of the oxide semiconductor can be suppressed. Thecrystallization of the oxide semiconductor is suppressed, wherebycharacteristics of the thin film transistor can be stabilized. Forexample, an In—Ga—Zn—O-based oxide semiconductor is made to contain theimpurity such as silicon oxide. Thus, crystallization of the oxidesemiconductor or generation of microcrystal grains can be prevented evenby heat treatment at 300° C. to 600° C.

In a manufacturing process of a thin film transistor in which anIn—Ga—Zn—O-based oxide semiconductor layer is a channel formationregion, an S value (a subthreshold swing value) or field effect mobilitycan be improved by heat treatment. Even in such a case, crystallizationand generation of microcrystal grains can be prevented as describedabove, whereby the thin film transistor can be prevented from beingnormally-on. Further, even in the case where heat stress or bias stressis added to the thin film transistor, variations in a threshold voltagecan be prevented.

In the case where the island-shape semiconductor layer 124 is formedusing an In—Ga—Zn—O-based oxide semiconductor as an oxide semiconductormaterial, for example, a sputtering method using an oxide semiconductortarget containing In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1) can beemployed. The sputtering can be performed under the followingconditions, for example; the distance between the substrate 100 and thetarget is 30 mm to 500 mm; the pressure is 0.1 Pa to 2.0 Pa; directcurrent (DC) power supply is 0.25 kW to 5.0 kW; the temperature is 20°C. to 100° C.; the atmosphere is a rare gas atmosphere such as argon, anoxide atmosphere, or a mixed atmosphere of a rare gas such as argon andoxide.

Alternatively, in the case where the island-shape semiconductor layer124 is formed using an In—Ga—Zn—O-based oxide semiconductor by asputtering method, insulating impurity may be contained in the oxidesemiconductor target containing In, Ga, and Zn. The impurity isinsulating oxide typified by silicon oxide, germanium oxide, aluminumoxide, or the like; insulating nitride typified by silicon nitride,aluminum nitride, or the like; or insulating oxynitride typified bysilicon oxynitride or aluminum oxynitride. For example, SiO₂ ispreferably contained at a percentage of 0.1 wt % to 10 wt %, morepreferably a percentage of 1 wt % to 6 wt % in the oxide semiconductortarget. Insulating impurity is contained in the oxide semiconductor,whereby the oxide semiconductor to be formed is easily made amorphous.Further, when heat treatment is performed on the oxide semiconductorfilm, the oxide semiconductor film can be prevented from beingcrystallized.

In this embodiment, the case where the island-shape semiconductor layer124 using an oxide semiconductor material having a single layer isformed is described; however, the island-shape semiconductor layer 124may have a stacked-layer structure. For example, a semiconductor layer(hereinafter called a “semiconductor layer with high conductivity”)having the same constituent element as and a different constituent ratiothereof from the above semiconductor layer 124 is formed over theconductive layer 116. When etching in which a source electrode and adrain electrode are formed is performed, the semiconductor layer isetched, and after that, a semiconductor layer (hereinafter called a“semiconductor layer with normal conductivity”) having the sameconstituent as the above semiconductor layer 124 is formed. Thus, thisstructure can be employed instead of the above structure. In this case,since the semiconductor layer with high conductivity is provided betweenthe source electrode (or the drain electrode) and the semiconductorlayer with normal conductivity, element characteristics can be improved.

Film formation conditions of the semiconductor layer with highconductivity and the semiconductor layer with normal conductivity arepreferably different. For example, a flow rate ratio of an oxygen gas toan argon gas in the film formation conditions of the semiconductor layerwith high conductivity is smaller than that in the film formationconditions of the semiconductor layer with normal conductivity.Specifically, the semiconductor layer with high conductivity is formedin a rare gas (such as argon or helium) atmosphere or an atmospherecontaining an oxygen gas at 10% or less and a rare gas at 90% or more.The semiconductor layer with normal conductivity is formed in an oxygenatmosphere or an atmosphere in which a flow rate of an oxygen gas is 1time or more that of a rare gas. In such a manner, two kinds ofsemiconductor layers having different conductivities can be formed.

Note that a pulse direct current (DC) power supply is preferably usedbecause dust can be reduced and the film thickness can be uniform.Further, in the case where the island-shape semiconductor layer 124 isformed without being exposed to the air after the above-described plasmatreatment, dust or moisture can be prevented from being attached to aninterface between the gate insulating layer 114 and the island-shapesemiconductor layer 124. In addition, attachment of impurities tosurfaces of the source electrode 118 and the drain electrode 120,oxidation of the surfaces, or the like can be suppressed. Note that thethickness of the island-shape semiconductor layer 124 may be about 5 nmto 200 nm.

As the above sputtering method, an RF sputtering method in which ahigh-frequency power source is used for a sputtering power source, a DCsputtering method in which a direct current power source is used, apulse DC sputtering method in which a direct-current bias is applied ina pulse manner, or the like can be employed.

Through the above steps, a transistor 150 in which the island-shapesemiconductor layer 124 is used as a channel formation region can beformed. Further, in a region where a second wiring 122 is overlappedwith a first wiring 110 (a region where the first wiring 110 and thesecond wiring 122 intersect with each other), a stacked-layer structure152 of the first wiring 110, the resist mask 112, the gate insulatinglayer 114, and the second wiring 122 can be formed. Thus, a capacitancevalue of parasitic capacitance can be reduced while suppressing increasein the number of manufacturing steps.

Note that heat treatment at 100° C. to 800° C., typically 200° C. to400° C., is preferably performed after the island-shape semiconductorlayer 124 using an oxide semiconductor material is formed. For example,heat treatment can be performed at 350° C. for an hour in a nitrogenatmosphere. Through this heat treatment, rearrangement at the atomiclevel of the In—Ga—Zn—O-based oxide semiconductor included in theisland-shape semiconductor layer 124 occurs. This heat treatment(including photo-annealing and the like) is important in terms ofreleasing distortion which interrupts carrier movement in theisland-shape semiconductor layer 124. Note that there is no particularlimitation on the timing of the above heat treatment as long as it isafter the island-shape semiconductor layer 124 (or the semiconductorlayer before the etching) is formed.

The island-shape semiconductor layer 124 using an oxide semiconductormaterial may be subjected to oxygen radical treatment. The transistor150 is easily normally off by oxygen radical treatment. In addition, theradical treatment can repair damage due to the etching of theisland-shape semiconductor layer 124. The radical treatment can beperformed in an atmosphere of O₂, N₂O, N₂ containing oxygen, He, Ar, orthe like. Alternatively, the radical treatment may be performed in anatmosphere in which Cl₂ and CF₄ are added to the above atmosphere. Notethat the radical treatment is preferably performed without applicationof bias voltage to the substrate 100 side.

After that, a protective insulating layer (not shown) is formed so as tocover the transistor 150 and the stacked-layer structure 152. Theprotective insulating layer may be formed by a single layer or a stackedlayer of a film formed of a material such as silicon oxide, siliconnitride, silicon oxynitride, silicon nitride oxide, aluminum oxide,aluminum nitride, aluminum oxynitride, or aluminum nitride oxide by aCVD method, a sputtering method, or the like. Alternatively, theprotective insulating layer may be formed by a film formed of an organicmaterial having heat resistance such as polyimide, acrylic,benzocyclobutene, polyamide, or epoxy by a spin coating method, adroplet discharge method (e.g., an ink-jet method, screen printing,offset printing), or the like. In addition to such organic materials, alow-dielectric constant material (a low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like can be used as well. Note that a siloxane-based resin is aresin formed from a siloxane-based material as a starting material andhaving the bond of Si—O—Si. As a substituent, an organic group (e.g., analkyl group or an aryl group) or a fluoro group may be used. The organicgroup may include a fluoro group.

After that, a variety of electrodes and a wiring are formed, whereby asemiconductor device provided with the transistor 150 is completed.

As described in this embodiment, part of the resist mask formed using amulti-tone mask is provided between the first wiring and the secondwiring, whereby the capacitance value of the parasitic capacitance canbe reduced while suppressing increase in the number of manufacturingsteps.

Note that this embodiment can be implemented in combination with any ofthe other embodiments or example as appropriate.

Embodiment 2

In this embodiment, an example, which is different from the aboveembodiment, of a method for manufacturing a semiconductor device isdescribed with reference to drawings. Note that many parts of a step ofmanufacturing a semiconductor device in this embodiment are the same asthose in the other embodiments. Therefore, hereinafter, description forthe same parts as those of the above embodiment is omitted and differentparts from the above embodiment are described in detail.

First, the conductive layer 102 is formed over the substrate 100 and theresist masks 104 and 105 are selectively formed over the conductivelayer 102 (see FIG. 3A). Note that in this embodiment, the resist mask104 and the resist mask 105 are the almost same thickness.

Embodiment 1 can be referred to for the details of the substrate 100 andthe conductive layer 102; therefore description thereof is omitted here.

The resist masks 104 and 105 can be manufactured without using anyspecial method. Needless to say, a multi-tone mask may be used, or anink-jet method may be used.

Next, the conductive layer 102 is etched using the resist masks 104 and105, so that the gate electrode 108 and a first wiring 109 are formed(see FIG. 3B).

Embodiment 1 can also be referred to for the detail of the above etchingtreatment. Note that after the above etching treatment, the resist masks104 and 105 are removed.

Next, an insulating layer 111 is formed so as to cover the gateelectrode 108 and the first wiring 109 (see FIG. 3C). The gateinsulating layer 111 can be formed using a material such as siliconoxide, silicon oxynitride, silicon nitride, silicon nitride oxide,aluminum oxide, and tantalum oxide. Alternatively, an organic materialhaving heat resistance, such as polyimide, acrylic, benzocyclobutene,polyamide, or epoxy can be used. In addition to such organic materials,a low-dielectric constant material (a low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like can be used. Alternatively, the insulating layer 111 may beformed by stacking films formed of these materials. In particular, alow-dielectric constant material is preferably used because theparasitic capacitance can be effectively reduced. These films are formedto a thickness of greater than or equal to 50 nm, preferably greaterthan or equal to 200 nm, more preferably greater than or equal to 500 nmby a sputtering method or the like. For example, a silicon oxide filmcan be formed to a thickness of 250 nm by a sputtering method as theinsulating layer 111.

Next, the above insulating layer 111 is selectively etched to form aninsulating layer 113 covering the first wiring 109 (see FIG. 3D). As theabove etching treatment, dry etching may be used, or wet etching may beused. By the etching treatment, a surface of the gate electrode 108 isexposed.

Next, the gate insulating layer 114 is formed so as to cover the gateelectrode 108, the insulating layer 113, and the like (see FIG. 4A).Embodiment 1 can be referred to for the detail of the gate insulatinglayer 114.

Next, the conductive layer 116 is formed over the gate insulating layer114 (see FIG. 4B). The conductive layer 116 can be formed using amaterial and by a method which are similar to those of the conductivelayer 102. For example, the conductive layer 116 can be formed to have asingle-layer structure of a molybdenum film or a titanium film.Alternatively, the conductive layer 116 may be formed to have astacked-layer structure and can have a stacked-layer structure of analuminum film and a titanium film, for example. A three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order may be employed. A three-layer structure in whicha molybdenum film, an aluminum film, and a molybdenum film are stackedin this order may be employed. Further, an aluminum film containingneodymium (an Al—Nd film) may be used as the aluminum film used forthese stacked-layer structures. Further alternatively, the conductivelayer 116 may have a single-layer structure of an aluminum filmcontaining silicon.

Next, the conductive layer 116 is selectively etched to form the sourceelectrode 118, the drain electrode 120, and the second wiring 122 (seeFIG. 4C).

Although not described in this embodiment, after the above steps, thegate insulating layer 114, the source electrode 118, and the drainelectrode 120 may be subjected to surface treatment. As the surfacetreatment, plasma treatment using an inactive gas and/or a reactive gasor the like can be performed. Embodiment 1 can be referred to for thedetail of the plasma treatment.

Next, after a semiconductor layer is formed so as to cover the gateinsulating layer 114, the source electrode 118, and the drain electrode120, the semiconductor layer is selectively etched, so that theisland-shape semiconductor layer 124 is formed in which at least partthereof is in contact with the source electrode 118 and the drainelectrode 120 (see FIG. 4D). Embodiment 1 may be referred to for thedetail of the island-shape semiconductor layer 124. Note that in thisembodiment, the case where an oxide semiconductor material is used asthe island-shape semiconductor layer 124 is described.

Note that also in this embodiment, the semiconductor layer can have astacked-layer structure as described in Embodiment 1. The semiconductorlayer with high conductivity is provided in a portion which is incontact with the source electrode (or the drain electrode), wherebyelement characteristics can be improved.

Besides, Embodiment 1 can be referred to for the detail of forming theisland-shape semiconductor layer 124. Embodiment 1 can be referred tofor the details of a variety of treatment on the island-shapesemiconductor layer 124 as well.

Through the above steps, a transistor 160 in which the island-shapesemiconductor layer 124 is used as a channel formation region can beformed. Further, in a region where the second wiring 122 is overlappedwith the first wiring 109 (a region where the first wiring 109 and thesecond wiring 122 intersect with each other), a stacked-layer structure162 of the first wiring 109, the insulating layer 113, the gateinsulating layer 114, and the second wiring 122 can be formed. Thus, thecapacitance value of the parasitic capacitance can be reduced.

After that, a protective insulating layer (not shown) is formed so as tocover the transistor 160 and the stacked-layer structure 162. Embodiment1 can be referred to for the details. Then, a variety of electrodes anda wiring are formed, whereby a semiconductor device provided with thetransistor 160 is completed.

As described in this embodiment, an insulating layer other than a gateinsulating layer is provided between the first wiring and the secondwiring, whereby the capacitance value of the parasitic capacitance canbe reduced without increasing the thickness of the gate insulatinglayer. In other words, the capacitance value of the parasiticcapacitance can be reduced without deteriorating elementcharacteristics.

Note that this embodiment can be implemented in combination with any ofthe other embodiments or example as appropriate.

Embodiment 3

In this embodiment, an example, which is different from the aboveembodiments, of a step of manufacturing a semiconductor device isdescribed with reference to drawings. Note that many parts of a methodfor manufacturing a semiconductor device in this embodiment are the sameas those in the other embodiments. Therefore, description for the sameparts as those of the above embodiments is omitted and different partsfrom the above embodiments are described in detail.

First, the conductive layer 102 is formed over the substrate 100 and theresist masks 104 and 105 are selectively formed over the conductivelayer 102 (see FIG. 5A). Note that in this embodiment, the resist mask104 and the resist mask 105 are the almost same thickness.

Embodiment 1 can be referred to for the details of the substrate 100 andthe conductive layer 102; therefore description thereof is omitted here.

The resist masks 104 and 105 can be manufactured without using anyspecial method. Needless to say, a multi-tone mask may be used, or anink-jet method may be used.

Next, the conductive layer 102 is etched using the resist masks 104 and105, so that the gate electrode 108 and the first wiring 109 are formed(see FIG. 5B).

Embodiment 1 can also be referred to for the detail of the above etchingtreatment. Note that after the above etching treatment, the resist masks104 and 105 are removed.

Next, the gate insulating layer 114, the insulating layer 115, theconductive layer 116, and a semiconductor layer 117 with highconductivity are stacked in this order so as to cover the gate electrode108 and the first wiring 109 (see FIG. 5C).

Embodiment 1 or the like can be referred to for the details of the gateinsulating layer 114 and the conductive layer 116. The detail of theinsulating layer 111 in Embodiment 2 can be referred to for theinsulating layer 115. In addition, the semiconductor layer 117 with highconductivity corresponds to the “semiconductor layer with highconductivity” in Embodiment 1 or the like.

A combination of the gate insulating layer 114 and the insulating layer115 is preferably a combination in which a selectivity ratio in etchingwhich is a later step can be obtained. For example, when silicon oxideand silicon nitride are combined, the selectivity ratio in etching canbe preferably obtained. In this embodiment, the case where the gateinsulating layer 114 is formed using silicon oxide and the insulatinglayer 115 is formed using silicon nitride is described.

The semiconductor layer 117 with high conductivity can be formed, forexample, by a sputtering method using an oxide semiconductor targetcontaining In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1). The sputtering can beperformed under the following conditions, for example; the distancebetween the substrate 100 and the target is 30 mm to 500 mm; thepressure is 0.1 Pa to 2.0 Pa; direct current (DC) power supply is 0.25kW to 5.0 kW; the temperature is 20° C. to 100° C.; the atmosphere is arare gas atmosphere such as argon, or a mixed atmosphere of a rare gassuch as argon and oxide.

More specifically, the above semiconductor layer 117 with highconductivity is preferably formed under a condition where the flow rateof oxygen is small. For example, the atmosphere can be a rare gas (suchas argon or helium) atmosphere or an atmosphere containing an oxygen gasat 10% or less and a rare gas at 90% or more. Thus, the oxygenconcentration of the film formation atmosphere is reduced, whereby asemiconductor layer with high conductivity can be obtained.

In the above description, the case where an oxide semiconductor materialis used for the semiconductor layer of the transistor is described as anexample; however, a semiconductor material such as silicon, germanium,silicon germanium, silicon carbide, gallium arsenide, or indiumphosphide may be used. For example, in the case of using silicon for thesemiconductor layer of the transistor, the semiconductor layer 117 withhigh conductivity can be formed using a material in which phosphorus(P), boron (B), or the like is added to silicon.

The semiconductor layer 117 with high conductivity is provided, wherebyelement characteristics can be improved. However, the semiconductorlayer 117 with high conductivity is not a necessary component and can beomitted as appropriate.

Next, The insulating layer 115, the conductive layer 116, and thesemiconductor layer 117 with high conductivity are selectivity etched,so that the source electrode 118, a semiconductor layer 119 with highconductivity, the drain electrode 120, a semiconductor layer 121 withhigh conductivity, the second wiring 122, and a semiconductor layer 123with high conductivity are formed (see FIG. 5D).

As described above, the etching treatment is preferably performed undera condition in which the insulating layer 115 is etched more easily thanthe gate insulating layer 114 can be obtained. It is extremely importantto perform the etching treatment under the condition where theinsulating layer 115 is etched more easily than the gate insulatinglayer 114 can be obtained. The reason for this is as follows. Thethickness of the insulating layer 115 is larger than the thickness ofthe gate insulating layer 114. In the case where etching treatment isperformed under the condition where the insulating layer 115 is etchedless easily than the gate insulating layer 114, variations in thicknessof the gate insulating layer 114 due to the etching of the gateinsulating layer 114 are caused, and there is a concern that elementcharacteristics are deteriorated. Note that there is no particularlimitation on the etching treatment other than the above condition.

Next, after a semiconductor layer is formed so as to cover the gateinsulating layer 114, the source electrode 118, the semiconductor layer119 with high conductivity, the drain electrode 120, and thesemiconductor layer 121 with high conductivity, the semiconductor layeris selectively etched, so that the island-shape semiconductor layer 124is formed in which at least part thereof is in contact with thesemiconductor layer 119 with high conductivity and the semiconductorlayer 121 with high conductivity (see FIG. 5E). Embodiment 1 may bereferred to for the detail of the island-shape semiconductor layer 124.

Besides, Embodiment 1 can be referred to for the detail of forming theisland-shape semiconductor layer 124. Embodiment 1 can be referred tofor the details of a variety of treatment on the island-shapesemiconductor layer 124 as well.

Through the above steps, a transistor 170 in which the island-shapesemiconductor layer 124 is used as a channel formation region can beformed. Further, in the region where the second wiring 122 is overlappedwith the first wiring 109 (the region where the first wiring 109 and thesecond wiring 122 intersect with each other), a stacked-layer structure172 of the first wiring 109, the gate insulating layer 114, theinsulating layer 115, the second wiring 122, and the semiconductor layer123 with high conductivity can be formed. Thus, the capacitance value ofthe parasitic capacitance can be reduced.

After that, a protective insulating layer (not shown) is formed so as tocover the transistor 170 and the stacked-layer structure 172. Embodiment1 can be referred to for the details. Then, a variety of electrodes anda wiring are formed, whereby a semiconductor device provided with thetransistor 170 is completed.

As described in this embodiment, an insulating layer other than a gateinsulating layer is provided between the first wiring and the secondwiring, whereby the capacitance value of the parasitic capacitance canbe reduced without increasing the thickness of the gate insulatinglayer. In other words, the capacitance value of the parasiticcapacitance can be reduced without deteriorating elementcharacteristics. In addition, etching treatment of the insulating layerand the gate insulating layer is performed under the condition where theselectivity ratio can be obtained, so that a semiconductor device inwhich variations in element characteristics are suppressed can beprovided.

Note that this embodiment can be implemented in combination with any ofthe other embodiments or example as appropriate.

Embodiment 4

In this embodiment, a step of manufacturing of an active matrixsubstrate which is an example of a usage pattern of a semiconductordevice is described with reference to drawings. Note that many parts ofa manufacturing step described in this embodiment are the same as thosein Embodiments 1 to 3. Therefore, hereinafter, description for the sameparts as those of the above embodiments is omitted and different partsfrom the above embodiments are described in detail. Note that in thefollowing description, FIGS. 6A to 6C and FIGS. 7A to 7C arecross-sectional views and FIG. 8 is a plan view. In addition, A1-A2,B1-B2, and C1-C2 in FIGS. 6A to 6C and FIGS. 7A to 7C are regionscorresponding to A1-A2, B1-B2, and C1-C2 in FIG. 8, respectively.

First, a wiring and an electrode (a gate electrode 202, a capacitorwiring 204, a first wiring 206, and a first terminal 208) are formedover a substrate 200 having an insulating surface (see FIG. 6A). Notethat the gate electrode 202 and the first wiring 206 are illustrateddistinctively in the drawing for convenience in order to clarify anintersection portion of the wirings; however, it is needless to say thata structure may be used in which the gate electrode 202 and the firstwiring 206 are integrated.

In this embodiment, the case where the method described in Embodiment 1,in other words, the case where the above wirings and electrode areformed using a multi-tone mask is described. Specifically, after theabove wirings and electrode are formed, a resist mask is made to recede,so that a resist mask 210 is left over part of the first wiring 206 (seeFIG. 6A). Embodiment 1 can be referred to for a method for forming theresist mask, a method for making the resist mask recede, and the like.

Note that the capacitor wiring 204 and the first terminal 208 can beformed at the same time using the same material and the samemanufacturing method as the gate electrode 202. Embodiment 1 can bereferred to for the details of the material and the manufacturing methodof the gate electrode 202.

Next, a gate insulating layer 212 is formed over the gate electrode 202and the gate insulating layer 212 is selectively etched so as to exposethe first terminal 208, whereby a contact hole is formed (see FIG. 6B).There is no particular limitation on the etching treatment. Wet etchingmay be used, or dry etching may be used.

Next, after a conductive layer covering the gate insulating layer 212and the first terminal 208 is formed, the conductive layer isselectively etched, so that a source electrode 214 (or a drainelectrode), a drain electrode 216 (or a source electrode), a secondwiring 218, a connection electrode 220, and a second terminal 222 areformed (see FIG. 6C). Note that the source electrode 214 and the secondwiring 218 are illustrated distinctively in the drawing for conveniencein order to clarify an intersection portion of the wirings; however, itis needless to say that a structure may be used in which the sourceelectrode 214 and the second wiring 218 are integrated.

The detail of the conductive layer 102 in Embodiment 1 or the like canbe referred to for the material and the manufacturing method of theabove conductive layer. There is no particular limitation on etchingtreatment; however, in the case of using dry etching treatment,miniaturization of a wiring structure can be achieved as compared to thecase of using wet etching treatment.

For example, the connection electrode 220 can be in directly contactwith the first terminal 208 through a contact hole formed in the gateinsulating layer 212. Further, the second terminal 222 can beelectrically connected to the second wiring 218 (including the sourceelectrode 214).

Next, after a semiconductor layer is formed so as to cover at least thesource electrode 214 and the drain electrode 216, the semiconductorlayer is selectively etched to form the island-shape semiconductor layer224 (see FIG. 7A). Here, the island-shape semiconductor layer 224 is incontact with parts of the source electrode 214 and the drain electrode216. Embodiment 1 can be referred to for the detail of the island-shapesemiconductor layer 224 as well. Note that also in this embodiment, thecase where the island-shape semiconductor layer 124 using an oxidesemiconductor material is formed to have a single-layer structure isdescribed.

Note that heat treatment at 100° C. to 800° C., typically 200° C. to400° C., is preferably performed after the island-shape semiconductorlayer 224 using an oxide semiconductor material is formed. For example,heat treatment can be performed at 350° C. for an hour in a nitrogenatmosphere. There is no particular limitation on the timing of the heattreatment as long as it is after the island-shape semiconductor layer224 (or the semiconductor layer before the etching) is formed.Embodiment 1 or the like can be referred to for the detail of the othertreatment.

Through the above steps, a transistor 250 is completed.

Next, a protective insulating layer 226 covering the transistor 250 isformed and the protective insulating layer 226 is selectively etched toform a contact hole reaching the drain electrode 216, the connectionelectrode 220, and the second terminal 222 (see FIG. 7B).

Next, transparent conductive layers 228, 230, and 232 which areelectrically connected to the drain electrode 216, the connectionelectrode 220, and the second terminal 222, respectively, are formed(see FIG. 7C and FIG. 8).

The transparent conductive layer 228 functions as a pixel electrode andthe transparent conductive layers 230 and 232 function as an electrodeor a wiring used for connection with a flexible printed circuit (anFPC). More specifically, the transparent conductive layer 230 formedover the connection electrode 220 can be used as a terminal electrodefor connection which functions as an input terminal for the gate wiring(the first wiring 206 in this embodiment) and the transparent conductivelayer 232 formed over the second terminal 222 can be used as a terminalelectrode for connection which functions as an input terminal for thesource wiring (the second wiring 218 in this embodiment).

In addition, storage capacitance can be formed by the capacitor wiring204, the gate insulating layer 212, and the transparent conductive layer228.

The transparent conductive layers 228, 230, and 232 can be formed usinga material such as indium oxide (In₂O₃), indium oxide tin oxide alloy(In₂O₃—SaO₂, abbreviated as ITO), or indium oxide zinc oxide alloy(In₂O₃—ZnO). For example, after the films containing the above materialare formed by a sputtering method, a vacuum evaporation method, or thelike, an unnecessary portion is removed by etching, whereby thetransparent conductive layers 228, 230, and 232 may be formed.

Through the above steps, an active matrix substrate including abottom-gate transistor and an element such as storage capacitance can becompleted. For example, in the case of manufacturing an active matrixliquid crystal display device by using this, a liquid crystal layer maybe provided between an active matrix substrate and a counter substrateprovided with a counter electrode, and the active matrix substrate andthe counter substrate may be fixed to each other.

As described in this embodiment, part of the resist mask formed using amulti-tone mask is provided between the first wiring and the secondwiring, whereby the capacitance value of the parasitic capacitance canbe reduced while suppressing increase in the number of manufacturingsteps.

In this embodiment, the method for manufacturing an active matrixsubstrate is described in accordance with the method described inEmbodiment 1; however, the present invention disclosed is not limitedthereto. An active matrix substrate may be manufactured by the methoddescribed in Embodiment 2 or 3. Note that this embodiment can beimplemented in combination with any of the other embodiments or exampleas appropriate.

Embodiment 5

In this embodiment, another example of a step of manufacturing of anactive matrix substrate is described with reference to drawings. Notethat many parts of a method for manufacturing a semiconductor device inthis embodiment are the same as those in Embodiments 1 to 4. Therefore,hereinafter, description for the same parts as those of the aboveembodiments is omitted and different parts from the above embodimentsare described in detail. Note that in the following description, FIGS.9A to 9C are cross-sectional views and FIG. 10 is a plan view. Inaddition, A1-A2, B1-B2, and C1-C2 in FIGS. 9A to 9C are regionscorresponding to A1-A2, B1-B2, and C1-C2 in FIG. 10, respectively.

First, a conductive layer is formed over the substrate 200 having aninsulating surface and a resist mask 209 is formed over the conductivelayer using a multi-tone mask. The conductive layer is etched using theresist mask 209 to form conductive layers 201, 203, 205, and 207 (seeFIG. 9A).

Embodiments 1 to 4 can be referred to for the details of the conductivelayers and the resist mask. Note that in the above etching, theconductive layers 201, 203, and 207 are formed thicker than an electrodeor the like which is finally formed. In addition, the width of theconductive layer 205 in C1-C2 is smaller than the width thereof in theother regions.

Next, after the resist mask 209 is made to recede to expose surfaces ofthe conductive layers 201, 203, and 207, the gate electrode 202, thecapacitor wiring 204, the first wiring 206, and the first terminal 208are formed by thinning treatment (see FIG. 9B). At a stage in which theresist mask 209 is made to recede, the resist mask 210 is partly leftabove part of the conductive layer 205. Thus, only the region where theresist mask 210 is not left is thinned.

As the thinning treatment, a variety of etching treatment can be used.Note that the width of the first wiring 206 is slightly smaller than thewidth of the conductive layer 205 due to the etching treatment.

After that, the gate insulating layer 212, the source electrode 214, thedrain electrode 216, the second wiring 218, the connection electrode220, the second terminal 222, the island-shape semiconductor layer 224,the protective insulating layer 226, the transparent conductive layers228, 230, 232, and 234, and the like are formed, whereby an activematrix substrate is completed (see FIG. 9C and FIG. 10). Embodiment 4 orthe like may be referred to for steps after the step of forming the gateinsulating layer 212. Note that in this embodiment, when the transparentconductive layer 228 and the like are formed, the transparent conductivelayer 234 is also formed in a region overlapping with the first wiring206 over the second wiring 218.

In this embodiment, the widths of the first wiring 206 and the secondwiring 218 are reduced in a region where the first wiring 206 and thesecond wiring 218 intersect with each other. Thus, the capacitance valueof the parasitic capacitance formed in an intersection region of thewirings can be further reduced. In the region where the first wiring 206and the second wiring 218 intersect with each other, the first wiring206 is formed thick, and the transparent conductive layer 234 isprovided over the second wiring 218. Thus, the increase in wiringresistance due to the decrease in the wiring width can be prevented andthe decrease in performance of a semiconductor device can be suppressed.

Note that in this embodiment, a structure where the width and thethickness of the wiring in the region where the first wiring 206 and thesecond wiring 218 intersect with each other are different from those ofwirings in the other regions is employed; however, the present inventiondisclosed is not limited thereto. Also in an intersection region of thecapacitor wiring 204 and the second wiring 218, a structure similar tothe above structure can be employed. In this case, the capacitance valueof the parasitic capacitance which occurs in the intersection region ofthe capacitor wiring 204 and the second wiring 218 can also be reduced.

This embodiment can be implemented in combination with any of the otherembodiments or example as appropriate.

Embodiment 6

In this embodiment, the case where a thin film transistor ismanufactured and a semiconductor device having a display function (alsoreferred to as a display device) is manufactured using the thin filmtransistor in a pixel portion and in a driver circuit is described.Further, part or whole of a driver circuit can be formed over the samesubstrate as a pixel portion, whereby a system-on-panel can be obtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement), a light-emitting element (also referred to as a light-emittingdisplay element), or the like can be used. Light-emitting elementsinclude, in its category, an element whose luminance is controlled bycurrent or voltage, and specifically include an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Further, a display medium whose contrast is changed by an electriceffect, such as electronic ink, may be used.

Further, the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel. Furthermore, an element substratewhich forms a display device is provided with means for supplyingcurrent to the display element in each of pixel portions. Specifically,the element substrate may be in a state after only a pixel electrode ofthe display element is formed, or a state after a conductive film to bea pixel electrode is formed and before the conductive film is etched.

Note that a display device in this specification means an image displaydevice, a display device, a light source (including a lighting device),and the like. Further, the display device also includes the followingmodules in its category: a module to which a connector such as an FPC(flexible printed circuit), a TAB (tape automated bonding) tape, or aTCP (tape carrier package) is attached; a module having a TAB tape or aTCP at the tip of which a printed wiring board is provided; a module inwhich an IC (integrated circuit) is directly mounted on a displayelement by a COG (chip on glass) method, and the like.

Hereinafter, in this embodiment, an example of a liquid crystal displaydevice is described. FIGS. 11A-1, 11A-2, and 11B are plan views and across-sectional view of a panel in which thin film transistors 4010 and4011 and a liquid crystal element 4013 which are formed over a firstsubstrate 4001 are sealed by a second substrate 4006 and a sealant 4005.Here, FIGS. 11A-1 and 11A-2 are each a plan view and FIG. 11B is across-sectional view taken along the line M-N of FIGS. 11A-1 and 11A-2.

The sealant 4005 is provided to surround a pixel portion 4002 and ascanning line driver circuit 4004 that are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scanning line driver circuit 4004. In other words,the pixel portion 4002 and the scanning line driver circuit 4004 aresealed together with a liquid crystal layer 4008, by the first substrate4001, the sealant 4005, and the second substrate 4006. Further, a signalline driver circuit 4003 that is formed using a single crystalsemiconductor film or a polycrystalline semiconductor film over asubstrate separately prepared is mounted in a region different from theregion surrounded by the sealant 4005 over the first substrate 4001.

Note that there is no particular limitation on the connection method ofa driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used as appropriate.FIG. 11A-1 illustrates an example of mounting the signal line drivercircuit 4003 by a COG method, and FIG. 11A2 illustrates an example ofmounting the signal line driver circuit 4003 by a TAB method.

In addition, the pixel portion 4002 and the scanning line driver circuit4004 provided over the first substrate 4001 each include a plurality ofthin film transistors. FIG. 11B illustrates the thin film transistor4010 included in the pixel portion 4002 and the thin film transistor4011 included in the scanning line driver circuit 4004. Insulatinglayers 4020 and 4021 are provided over the thin film transistors 4010and 4011.

As the thin film transistors 4010 and 4011, the thin film transistorswhich are described in Embodiments 1 to 5 or the like can be employed.Note that in this embodiment, the thin film transistors 4010 and 4011are n-channel thin film transistors.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. The liquid crystal element 4013 is formed by thepixel electrode layer 4030, the counter electrode layer 4031, and theliquid crystal layer 4008. Note that the pixel electrode layer 4030 andthe counter electrode layer 4031 are provided with an insulating layer4032 and an insulating layer 4033, respectively, each of which functionsas an alignment film. The liquid crystal layer 4008 is sandwichedbetween the pixel electrode layer 4030 and the counter electrode layer4031 with the insulating layers 4032 and 4033 interposed therebetween.

Note that as the first substrate 4001 and the second substrate 4006,glass, metal (typically, stainless steel), ceramic, plastic, or the likecan be used. As plastic, an FRP (fiberglass-reinforced plastics)substrate, a PVF (polyvinyl fluoride) film, a polyester film, an acrylicresin film, or the like can be used. Alternatively, a sheet with astructure in which an aluminum foil is sandwiched between PVF films orpolyester films can be used.

A columnar spacer 4035 is provided in order to control the distance (acell gap) between the pixel electrode layer 4030 and the counterelectrode layer 4031. The columnar spacer 4035 can be obtained byselective etching of an insulating film. Note that a spherical spacermay be used instead of a columnar spacer. Further, the counter electrodelayer 4031 is electrically connected to a common potential line providedover the same substrate as the thin film transistor 4010. For example,the counter electrode layer 4031 can be electrically connected to thecommon potential line through conductive particles provided between thepair of substrates. Note that the conductive particles are preferablycontained in the sealant 4005.

Alternatively, a liquid crystal showing a blue phase for which analignment film is unnecessary may be used. A blue phase is one of theliquid crystal phases, which is generated just before a cholestericphase changes into an isotropic phase while temperature of cholestericliquid crystal is increased. Since the blue phase is only generatedwithin a narrow range of temperatures, a liquid crystal compositioncontaining a chiral agent at 5 wt % or more is preferably used. Thus,the temperature range can be improved. The liquid crystal compositionwhich includes a liquid crystal showing a blue phase and a chiral agenthas a small response time of 10 μs to 100 μs, has optical isotropy,which makes the alignment process unneeded, and has a small viewingangle dependence.

Although an example of a transmissive liquid crystal display device isdescribed in this embodiment, the present invention is not limitedthereto. An embodiment of the present invention may also be applied to areflective liquid crystal display device or a semi-transmissive liquidcrystal display device.

In this embodiment, an example of the liquid crystal display device isdescribed in which a polarizing plate is provided on the outer surfaceof the substrate (on the viewer side) and a coloring layer and anelectrode layer used for a display element are provided on the innersurface of the substrate in this order; however, the polarizing platemay be provided on the inner surface of the substrate. The stacked-layerstructure of the polarizing plate and the coloring layer is not limitedto that described in this embodiment and may be set as appropriatedepending on materials of the polarizing plate and the coloring layer orconditions of manufacturing steps. Furthermore, a light-blocking filmserving as a black matrix may be provided.

In this embodiment, in order to reduce the surface roughness of the thinfilm transistor, the thin film transistor obtained in Embodiments 1 to 5is covered with the insulating layer 4021. Note that the insulatinglayer 4020 corresponds to the protective insulating layer in Embodiments1 to 5.

As the insulating layer 4021, an organic material having heat resistancesuch as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy can beused. In addition to such organic materials, a low-dielectric constantmaterial (a low-k material), a siloxane-based resin, PSG(phosphosilicate glass), BPSG (borophosphosilicate glass), or the likecan be used. Note that the insulating layer 4021 may be formed bystacking a plurality of insulating films formed of these materials.

Note that a siloxane-based resin is a resin formed from a siloxane-basedmaterial as a starting material and having the bond of Si—O—Si. As asubstituent, an organic group (e.g., an alkyl group or an aryl group) ora fluoro group may be used. The organic group may include a fluorogroup.

There is no particular limitation on the method for forming theinsulating layer 4021, and the insulating layer 4021 can be formed,depending on the material, by a sputtering method, an SOG method, a spincoating method, a dipping method, a spray coating method, a dropletdischarge method (an inkjet method, screen printing, offset printing, orthe like), a doctor knife, a roll coater, a curtain coater, a knifecoater, or the like.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe made of a light-transmitting conductive material such as indium oxidecontaining tungsten oxide, indium zinc oxide containing tungsten oxide,indium oxide containing titanium oxide, indium tin oxide containingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) may be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodemade of the conductive composition preferably has a sheet resistance of1.0×10⁴ Ω/sq. or less and a transmittance of 70% or more at a wavelengthof 550 nm. Furthermore, the resistivity of the conductive high moleculecontained in the conductive composition is preferably 0.1 Ω·cm or less.

As the conductive high molecule, a so-called π-electron conjugatedconductive high molecule can be used. For example, polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, or a copolymer of two or more kinds of them canbe given.

A variety of signals are supplied to the signal line driver circuit4003, the scanning line driver circuit 4004, the pixel portion 4002, orthe like from an FPC 4018.

In addition, a connection terminal electrode 4015 is formed from thesame conductive film as the pixel electrode layer 4030 included in theliquid crystal element 4013, and a terminal electrode 4016 is formedfrom the same conductive film as source and drain electrode layers ofthe thin film transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

Note that FIGS. 11A-1, 11A-2 and 11B illustrate an example in which thesignal line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001; however, the present invention disclosed is notlimited to this structure. The scanning line driver circuit may beseparately formed and then mounted, or only part of the signal linedriver circuit or part of the scanning line driver circuit may beseparately formed and then mounted.

FIG. 12 illustrates an example where a liquid crystal display modulewhich corresponds to one embodiment of a semiconductor device is formedusing a TFT substrate 2600.

In FIG. 12, the TFT substrate 2600 and a counter substrate 2601 arebonded to each other by a sealant 2602 and an element layer 2603including a TFT and the like, a liquid crystal layer 2604 including analignment film and a liquid crystal layer, a coloring layer 2605, apolarizing plate 2606, and the like are provided between the TFTsubstrate 2600 and the counter substrate 2601, whereby a display regionis formed. The coloring layer 2605 is necessary to perform colordisplay. In the case of the RGB system, respective coloring layerscorresponding to colors of red, green, and blue are provided forrespective pixels. Polarizing plates 2606 and 2607 and a diffusion plate2613 are provided outside the TFT substrate 2600 and the countersubstrate 2601. A light source includes a cold cathode tube 2610 and areflective plate 2611. A circuit board 2612 is connected to a wiringcircuit portion 2608 of the TFT substrate 2600 through a flexible wiringboard 2609. Thus, an external circuit such as a control circuit or apower source circuit is included in a liquid crystal module. Aretardation plate may be provided between the polarizing plate and theliquid crystal layer.

For a driving method of a liquid crystal, a TN (twisted nematic) mode,an IPS (in-plane-switching) mode, an FFS (fringe field switching) mode,an MVA (multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optical compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

Through the above steps, a high-performance liquid crystal displaydevice can be manufactured. Note that this embodiment can be implementedin combination with any of the other embodiments or example asappropriate.

Embodiment 7

In this embodiment, active matrix electronic paper which is an exampleof a semiconductor device is described with reference to FIG. 13. A thinfilm transistor 650 used for the semiconductor device can bemanufactured in a manner similar to that of the thin film transistordescribed in Embodiments 1 to 5.

The electronic paper in FIG. 13 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black or whiteare arranged between a first electrode layer and a second electrodelayer, and a potential difference is generated between the firstelectrode layer and the second electrode layer, whereby orientation ofthe spherical particles is controlled, so that display is performed.

The thin film transistor 650 provided over the substrate 600 is a thinfilm transistor of the present invention disclosed and has a structurein which an oxide semiconductor layer is sandwiched between the sourceor drain electrode layer which is above the oxide semiconductor layerand the source or drain electrode layers which is below the oxidesemiconductor layer. Note that the source or drain electrode layer iselectrically connected to a first electrode layer 660 through a contacthole formed in a protective insulating layer. A substrate 602 isprovided with a second electrode layer 670. Between the first electrodelayer 660 and the second electrode layer 670, spherical particles 680each having a black region 680 a and a white region 680 b are provided.A space around the spherical particles 680 is filled with a filler 682such as a resin (see FIG. 13). In Embodiment 13, the first electrodelayer 660 corresponds to a pixel electrode, and the second electrodelayer 670 corresponds to a common electrode. The second electrode layer670 is electrically connected to a common potential line provided overthe same substrate as the thin film transistor 650.

Instead of the twisting ball, an electrophoretic display element canalso be used. In that case, for example, a microcapsule having adiameter of approximately 10 μm to 200 μm in which transparent liquid,positively-charged white microparticles, and negatively-charged blackmicroparticles are encapsulated, is used. When an electric field isapplied between the first electrode layer and the second electrodelayer, the white microparticles and the black microparticles move toopposite sides from each other, so that white or black is displayed. Theelectrophoretic display element has higher reflectance than a liquidcrystal display element, and thus, an auxiliary light is unnecessary anda display portion can be recognized in a place where brightness is notsufficient. In addition, there is an advantage that even when power isnot supplied to the display portion, an image which has been displayedonce can be maintained.

Through the above steps, high-performance electronic paper can bemanufactured using the present invention disclosed. Note that thisembodiment can be implemented in combination with any of the otherembodiments or example as appropriate.

Embodiment 8

In this embodiment, an example of a light-emitting display device isdescribed as a semiconductor device. As a display element included in adisplay device, a light-emitting element utilizing electroluminescenceis described here. Light-emitting elements utilizing electroluminescenceare classified by whether a light-emitting material is an organiccompound or an inorganic compound. In general, the former is called anorganic EL element, and the latter is called an inorganic EL element.

In an organic EL element, by application of a voltage to alight-emitting element, electrons and holes are separately injected froma pair of electrodes into a layer containing a light-emitting organiccompound, and current flows. Then, the carriers (electrons and holes)recombine, thereby emitting light. Owing to such a mechanism, thelight-emitting element is called a current-excitation light-emittingelement.

The inorganic EL elements are classified into a dispersion-typeinorganic EL element and a thin-film-type inorganic EL element dependingon their element structures. A dispersion-type inorganic EL element hasa light-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination-type light emission which utilizes a donorlevel and an acceptor level. A thin-film-type inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized-type light emission that utilizesinner-shell electron transition of metal ions. Note that, here,description is made using an organic EL element as a light-emittingelement.

Structures of the light-emitting element are described with reference toFIGS. 14A to 14C. Here, a cross-sectional structure of a pixel isdescribed by taking an n-channel driving TFT as an example. TFTs 701,711, and 721 used for semiconductor devices illustrated in FIGS. 14A to14C can be manufactured in a manner similar to that of the thin filmtransistors described in Embodiments 1 to 5.

In order to extract light from a light-emitting element, at least one ofthe anode and the cathode is transparent. Here, transparent means thatat least an emission wavelength has sufficiently high transmittance. Asa method for extracting light, a thin film transistor and a lightemitting element are formed over a substrate; and there are a topemission method (a top extraction method) by which light is extractedfrom a side opposite to the substrate, a bottom emission method (abottom extraction method) by which light is extracted from the substrateside, a dual emission method (a dual extraction method) by which lightis extracted from both the substrate side and the side opposite to thesubstrate, and the like.

A light-emitting element having a top emission method is described withreference to FIG. 14A.

FIG. 14A is a cross-sectional view of a pixel in the case where light isemitted from a light-emitting element 702 to an anode 705 side. Here, acathode 703 of the light-emitting element 702 and the TFT 701 which is adriving TFT are electrically connected to each other, and alight-emitting layer 704 and the anode 705 are stacked in this orderover the cathode 703. As the cathode 703, a conductive film which has alow work function and reflects light can be used. For example, amaterial such as Ca, Al, CaF, MgAg, or AlLi is preferably used to formthe cathode 703. The light-emitting layer 704 may be formed using eithera single layer or a plurality of layers stacked. When the light-emittinglayer 704 is formed using a plurality of layers, an electron-injectinglayer, an electron-transporting layer, a light-emitting layer, ahole-transporting layer, and a hole-injecting layer are preferablystacked in this order over the cathode 703; however, needless to say, itis not necessary to form all of these layers. The anode 705 is formedusing a light-transmitting conductive material. For example, alight-transmitting conductive material such as indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium tin oxide (hereinafter referred to as ITO), indium zincoxide, or indium tin oxide to which silicon oxide is added may be used.

A structure in which the light-emitting layer 704 is sandwiched betweenthe cathode 703 and the anode 705 can be called the light-emittingelement 702. In the case of the pixel illustrated in FIG. 14A, light isemitted from the light-emitting element 702 to the anode 705 side asindicated by an arrow.

Next, a light-emitting element having a bottom emission method isdescribed with reference to FIG. 14B.

FIG. 14B is a cross-sectional view of a pixel in the case where light isemitted from a light-emitting element 712 to a cathode 713 side. Here,the cathode 713 of the light-emitting element 712 is formed over alight-transmitting conductive film 717 which is electrically connectedto the driving TFT 711, and a light-emitting layer 714 and an anode 715are stacked in this order over the cathode 713. Note that alight-blocking film 716 may be formed so as to cover the anode 715 whenthe anode 715 has a light-transmitting property. For the cathode 713, aconductive material having a low work function can be used like in thecase of FIG. 14A. Note that the cathode 713 is formed to a thicknessthat can transmit light (preferably, approximately 5 nm to 30 nm). Forexample, an aluminum film with a thickness of approximately 20 nm can beused as the cathode 713. Similarly to the case of FIG. 14A, thelight-emitting layer 714 may be formed using either a single layer or aplurality of layers stacked. Similarly to the case of FIG. 14A, theanode 715 is not required to transmit light, but may be made of alight-transmitting conductive material. As the light-blocking film 716,a metal which reflects light or the like can be used; however, it is notlimited thereto. For example, a resin to which black pigments is addedor the like can also be used.

A structure in which the light-emitting layer 714 is sandwiched betweenthe cathode 713 and the anode 715 can be called the light-emittingelement 712. In the case of the pixel illustrated in FIG. 14B, light isemitted from the light-emitting element 712 to the cathode 713 side asindicated by an arrow.

Next, a light-emitting element having a dual emission method isdescribed with reference to FIG. 14C.

In FIG. 14C, a cathode 723 of a light-emitting element 722 is formedover a light-transmitting conductive film 727 which is electricallyconnected to the driving TFT 721, and a light-emitting layer 724 and ananode 725 are stacked in this order over the cathode 723. For thecathode 723, a conductive material having a low work function can beused like in the case of FIG. 14A. Note that the cathode 723 is formedto a thickness that can transmit light. For example, an Al film with athickness of approximately 20 nm can be used as the cathode 723.Similarly to the case of FIG. 14A, the light-emitting layer 724 may beformed using either a single layer or a plurality of layers stacked.Similarly to the case of FIG. 14A, the anode 725 can be formed using alight-transmitting conductive material.

A structure where the cathode 723, the light-emitting layer 724, and theanode 725 overlap with one another can be called the light-emittingelement 722. In the case of the pixel illustrated in FIG. 14C, light isemitted from the light-emitting element 722 to both the anode 725 sideand the cathode 723 side as indicated by arrows.

Although an organic EL element is described here as a light-emittingelement, an inorganic EL element can also be provided as alight-emitting element. The example is described here in which a thinfilm transistor (a driving TFT) which controls the driving of alight-emitting element is electrically connected to the light-emittingelement; however, a structure may be employed in which a TFT for currentcontrol is connected between the driving TFT and the light-emittingelement.

Note that the structure of the semiconductor device described in thisembodiment is not limited to those illustrated in FIGS. 14A to 14C andcan be modified in various ways.

Next, the appearance and a cross section of a light-emitting displaypanel (also referred to as a light-emitting panel), which corresponds toone embodiment of the semiconductor device, are described with referenceto FIGS. 15A and 15B. FIGS. 15A and 15B are a plan view and across-sectional view of a panel in which thin film transistors 4509 and4510 and a light-emitting element 4511 which are formed over a firstsubstrate 4501 are sealed by a second substrate 4506 and a sealant 4505.FIG. 15A is a plan view and FIG. 15B is a cross-sectional view takenalong the line H-I of FIG. 15A.

A sealant 4505 is provided to surround a pixel portion 4502, signal linedriver circuits 4503 a and 4503 b, and scanning line driver circuits4504 a and 4504 b, which are provided over a first substrate 4501. Inaddition, a second substrate 4506 is provided over the pixel portion4502, the signal line driver circuits 4503 a and 4503 b, and thescanning line driver circuits 4504 a and 4504 b. In other words, thepixel portion 4502, the signal line driver circuits 4503 a and 4503 b,and the scanning line driver circuits 4504 a and 4504 b are sealedtogether with a filler 4507, by the first substrate 4501, the sealant4505, and the second substrate 4506. It is preferable that a displaydevice be thus packaged (sealed) using a protective film (such as abonding film or an ultraviolet curable resin film), a cover material, orthe like with high air-tightness and little degasification.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scanning line driver circuits 4504 a and 4504 b, which areformed over the first substrate 4501, each include a plurality of thinfilm transistors, and a thin film transistor 4510 included in the pixelportion 4502 and a thin film transistor 4509 included in the signal linedriver circuit 4503 a are illustrated as an example in FIG. 15B.

As the thin film transistors 4509 and 4510, the thin film transistorsdescribed in Embodiments 1 to 5 can be employed. Note that in thisembodiment, the thin film transistors 4509 and 4510 are n-channel thinfilm transistors.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 that is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a sourceelectrode layer or a drain electrode layer of the thin film transistor4510. In the structure of the light-emitting element 4511, the firstelectrode layer 4517, an electroluminescent layer 4512, and a secondelectrode layer 4513 are stacked; however, it is not limited to thestructure described in this embodiment. The structure of thelight-emitting element 4511 can be changed as appropriate depending onthe direction in which light is extracted from the light-emittingelement 4511, or the like.

A partition 4520 is formed using an organic resin film, an inorganicinsulating film, organic polysiloxane, or the like. It is particularlypreferable that the partition 4520 be formed of a photosensitivematerial to have an opening over the first electrode layer 4517 so thata sidewall of the opening is formed as an inclined surface withcontinuous curvature.

The electroluminescent layer 4512 may be formed using either a singlelayer or a plurality of layers stacked.

A protective film may be formed over the second electrode layer 4513 andthe partition 4520 in order to prevent oxygen, hydrogen, moisture,carbon dioxide, or the like from entering into the light-emittingelement 4511. As the protective film, a silicon nitride film, a siliconnitride oxide film, a DLC film, or the like can be formed.

A variety of signals are supplied to the signal line driver circuits4503 a and 4503 b, the scanning line driver circuits 4504 a and 4504 b,the pixel portion 4502, or the like from FPCs 4518 a and 4518 b.

In this embodiment, an example is described where a connection terminalelectrode 4515 is formed from the same conductive film as the firstelectrode layer 4517 of the light-emitting element 4511, and a terminalelectrode 4516 is formed from the same conductive film as the source anddrain electrode layers of the thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal of the FPC 4518 a through an anisotropic conductive film 4519.

The substrate located in the direction in which light is extracted fromthe light-emitting element 4511 needs to have a light-transmittingproperty. As a substrate having a light-transmitting property, a glassplate, a plastic plate, a polyester film, an acrylic film, and the likeare given.

As the filler 4507, an ultraviolet curable resin, a thermosetting resin,or the like can be used, in addition to an inert gas such as nitrogen orargon. For example, polyvinyl chloride (PVC), acrylic, polyimide, anepoxy resin, a silicone resin, polyvinyl butyral (PVB), ethylene vinylacetate (EVA), or the like can be used. In this embodiment, an examplewhere nitrogen is used for the filler is described.

If needed, an optical film, such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter, may be provided on a light-emitting surface of thelight-emitting element. Furthermore, an antireflection treatment may beperformed on a surface thereof. For example, anti-glare treatment bywhich reflected light can be diffused by projections and depressions onthe surface so as to reduce the glare can be performed.

The signal line driver circuits 4503 a and 4503 b and the scanning linedriver circuits 4504 a and 4504 b may be formed using a single crystalsemiconductor film or a polycrystalline semiconductor film over asubstrate separately prepared. Alternatively, only the signal linedriver circuits or part thereof, or only the scanning line drivercircuits or part thereof may be separately formed and mounted. Thisembodiment is not limited to the structure illustrated in FIGS. 15A and15B.

Through the above steps, a high-performance light-emitting displaydevice (display panel) can be manufactured. Note that this embodimentcan be implemented in combination with any of the other embodiments orexample as appropriate.

Embodiment 9

A semiconductor device can be applied to electronic paper. Electronicpaper can be used for electronic appliances of a variety of fields aslong as they can display data. For example, electronic paper can beapplied to an e-book reader (electronic book), a poster, anadvertisement in a vehicle such as a train, displays of various cardssuch as a credit card, or the like. Examples of the electronicappliances are illustrated in FIGS. 16A and 16B and FIG. 17.

FIG. 16A illustrates a poster 2631 using electronic paper. In the casewhere an advertising medium is printed paper, the advertisement isreplaced by hands; however, by using electronic paper to which anembodiment of the present invention is applied, the advertising displaycan be changed in a short time. Furthermore, stable images can beobtained without display defects. Note that the poster may have aconfiguration capable of wirelessly transmitting and receiving data.

FIG. 16B illustrates an advertisement 2632 in a vehicle such as a train.In the case where an advertising medium is printed paper, theadvertisement is replaced by hands; however, by using electronic paperto which an embodiment of the present invention is applied, theadvertising display can be changed in a short time with less manpower.Furthermore, stable images can be obtained without display defects. Notethat the advertisement in a vehicle may have a configuration capable ofwirelessly transmitting and receiving data.

FIG. 17 illustrates an example of an e-book reader 2700. For example,the e-book reader 2700 includes two housings, a housing 2701 and ahousing 2703. The housing 2701 and the housing 2703 are combined with ahinge 2711 so that the e-book reader 2700 can be opened and closed withthe hinge 2711 as an axis. With such a structure, the e-book reader 2700can be operated like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, text can bedisplayed on a display portion on the right side (the display portion2705 in FIG. 17) and graphics can be displayed on a display portion onthe left side (the display portion 2707 in FIG. 17).

FIG. 17 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, and the like may be provided onthe same surface as the display portion of the housing. Furthermore, anexternal connection terminal (an earphone terminal, a USB terminal, aterminal that can be connected to an AC adapter and various cables suchas a USB cable, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Moreover, the e-book reader 2700 may have a function of anelectronic dictionary.

The e-book reader 2700 may have a configuration capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

Note that this embodiment can be implemented in combination with any ofthe other embodiments or example as appropriate.

Embodiment 10

A semiconductor device can be applied to a variety of electronicappliances (including amusement machines). Examples of electronicappliances include television sets (also referred to as televisions ortelevision receivers), monitor of computers or the like, cameras such asdigital cameras or digital video cameras, digital photo frames, cellularphones (also referred to as mobile phones or mobile phone sets),portable game consoles, portable information terminals, audioreproducing devices, large-sized game machines such as pachinkomachines, and the like.

FIG. 18A illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. Images can be displayed on the display portion 9603. Here, thehousing 9601 is supported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with an operation key 9609 of the remote controller9610 so that an image displayed on the display portion 9603 can becontrolled. Furthermore, the remote controller 9610 may be provided witha display portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the receiver, a general television broadcast can bereceived. Furthermore, when the television set 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver, between receivers, or the like) datacommunication can be performed.

FIG. 18B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. Various images can be displayed on thedisplay portion 9703. For example, the display portion 9703 can displaydata of an image shot by a digital camera or the like to function as anormal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (a USB terminal, a terminalthat can be connected to various cables such as a USB cable, or thelike), a recording medium insertion portion, and the like. Although theymay be provided on the same surface as the display portion, it ispreferable to provide them on the side surface or the back surface forthe design of the digital photo frame 9700. For example, a memorystoring data of an image shot by a digital camera is inserted in therecording medium insertion portion of the digital photo frame, wherebythe image data can be downloaded and displayed on the display portion9703.

The digital photo frame 9700 may have a configuration capable ofwirelessly transmitting and receiving data. In this case, throughwireless communication, desired image data can be downloaded to bedisplayed.

FIG. 19A illustrates a portable amusement machine including twohousings: a housing 9881 and a housing 9891. The housings 9881 and 9891are connected to each other with a connection portion 9893 so as to beopened and closed. A display portion 9882 and a display portion 9883 areincorporated in the housing 9881 and the housing 9891, respectively. Inaddition, the portable amusement machine illustrated in FIG. 19Aincludes a speaker portion 9884, a recording medium insertion portion9886, an LED lamp 9890, an input means (an operation key 9885, aconnection terminal 9887, a sensor 9888 (a sensor having a function ofmeasuring force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,current, voltage, electric power, radiation, flow rate, humidity,gradient, oscillation, odor, or infrared rays), or a microphone 9889),and the like. Note that the structure of the portable amusement machineis not limited to the above and other structures provided with at leasta semiconductor device of an embodiment of the present invention may beemployed. The portable amusement machine illustrated in FIG. 19A has afunction of reading a program or data stored in a recording medium todisplay it on the display portion, and a function of sharing informationwith another portable amusement machine by wireless communication. Theportable amusement machine illustrated in FIG. 19A may have variousfunctions without limitation to the above.

FIG. 19B illustrates an example of a slot machine 9900 which is alarge-sized amusement machine. In the slot machine 9900, a displayportion 9903 is incorporated in a housing 9901. In addition, the slotmachine 9900 includes an operation means such as a start lever or a stopswitch, a coin slot, a speaker, and the like. Note that the structure ofthe slot machine 9900 is not limited to the above and other structuresprovided with at least a semiconductor device of an embodiment of thepresent invention may be employed.

FIG. 20A illustrates an example of a cellular phone 1000. The cellularphone 1000 is provided with a display portion 1002 incorporated in ahousing 1001, operation buttons 1003, an external connection port 1004,a speaker 1005, a microphone 1006, and the like.

When the display portion 1002 of the cellular phone 1000 illustrated inFIG. 20A is touched with a finger or the like, data can be input intothe cellular phone 1000. Furthermore, making calls, composing mails, orthe like can be performed by touching the display portion 1002 with afinger or the like.

There are mainly three screen modes of the display portion 1002. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing a mail, a textinput mode mainly for inputting text is selected for the display portion1002 so that text displayed on a screen can be input. In that case, itis preferable to display a keyboard or number buttons on almost all thearea of the screen of the display portion 1002.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 1000, display on the screen of the display portion 1002can be automatically switched by determining the direction of thecellular phone 1000 (whether the cellular phone 1000 is placedhorizontally or vertically for a landscape mode or a portrait mode).

The screen mode is switched by touching the display portion 1002,operating the operation buttons 1003 of the housing 1001, or the like.Alternatively, the screen mode can be switched depending on the kind ofimages displayed on the display portion 1002. For example, when a signalof an image displayed on the display portion is of moving image data,the screen mode is switched to the display mode. When the signal is oftext data, the screen mode is switched to the input mode.

Furthermore, in the input mode, when input by touching the displayportion 1002 is not performed for a certain period while a signal isdetected by the optical sensor in the display portion 1002, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 1002 can function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken bytouching the display portion 1002 with the palm or the finger, wherebypersonal authentication can be performed. Furthermore, by providing abacklight or a sensing light source emitting a near-infrared light forthe display portion, an image of a finger vein, a palm vein, or the likecan also be taken.

FIG. 20B illustrates another example of a cellular phone. The cellularphone in FIG. 20B has a display device 9410 and a communication device9400. The display device 9410 includes a housing 9411, a display portion9412, and operation buttons 9413. The communication device 9400 includesoperation buttons 9402, an external input terminal 9403, a microphone9404, a speaker 9405, and a light-emitting portion 9406 that emits lightwhen a phone call is received. The display device 9410 can be detachedfrom or attached to the communication device 9400 which has a phonefunction by moving in two directions represented by the allows. Thus,the display device 9410 and the communication device 9400 can beattached to each other along their short sides or long sides. Inaddition, when only the display function is needed, the display device9410 can be detached from the communication device 9400 and used alone.Images or input information can be transmitted or received by wirelessor wire communication between the communication device 9400 and thedisplay device 9410, each of which has a rechargeable battery.

Note that this embodiment can be implemented in combination with any ofthe other embodiments or example as appropriate.

Embodiment 11

In this embodiment, an example, which is different from the aboveembodiments, of a method for manufacturing a semiconductor device isdescribed with reference to drawings. Note that many parts of a step ofmanufacturing a semiconductor device in this embodiment are the same asthose in the other embodiments. Therefore, hereinafter, description forthe same parts as those of the above embodiments is omitted anddifferent parts from the above embodiments are described in detail.

First, the conductive layer 102 is formed over the substrate 100 and theresist masks 104 and 106 are selectively formed over the conductivelayer 102 (see FIG. 21A). The step is similar to the step in Embodiment1.

Next, after the conductive layer 102 is etched using the above resistmasks 104 and 106 to form the gate electrode 108 and the first wiring110, the resist masks 104 and 106 are made to recede to form the resistmask 112 over the first wiring 110, and the gate insulating layer 114 isformed so as to cover the resist mask 112, the gate electrode 108, andthe first wiring 110 which are formed (see FIG. 21B). The step is alsosimilar to the step in Embodiment 1; therefore, the detail is omitted.

Next, the conductive layer 116 and the semiconductor layer 180 with highconductivity are stacked in this order over the gate insulating layer114 (see FIG. 21C). The conductive layer 116 can be formed to have asingle-layer structure of a molybdenum film or a titanium film.Alternatively, the conductive layer 116 may be formed to have astacked-layer structure and can have a stacked-layer structure of analuminum film and a titanium film, for example. A three-layer structurein which a titanium film, an aluminum film, and a titanium film arestacked in this order may be employed. A three-layer structure in whicha molybdenum film, an aluminum film, and a molybdenum film are stackedin this order may be employed. Further, an aluminum film containingneodymium (an Al—Nd film) may be used as the aluminum film used forthese stacked-layer structures. Further alternatively, the conductivelayer 116 may have a single-layer structure of an aluminum filmcontaining silicon. The detail of the conductive layer 102 or the likein Embodiment 1 can be referred to for the detail of the conductivelayer 116.

There is no particular limitation on the semiconductor layer 180 withhigh conductivity as long as the semiconductor layer 180 with highconductivity has higher conductivity than an island-shape semiconductorlayer which is formed later. For example, in the case where theisland-shape semiconductor layer which is formed later is formed usingan oxide semiconductor material, a film formed of an oxide semiconductormaterial similar to that of the semiconductor layer with highconductivity can be formed under a different formation condition.Needless to say, the semiconductor layer 180 with high conductivity maybe formed using a different material from the island-shape semiconductorlayer which is formed later. In this embodiment, the case where thesemiconductor layer 180 with high conductivity and the island-shapesemiconductor layer which is formed later are formed using the samematerial is described.

In this embodiment, the semiconductor layer 180 with high conductivityis formed by a sputtering method using an oxide semiconductor targetcontaining In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1). The sputtering can beperformed under the following conditions, for example; the distancebetween the substrate 100 and the target is 30 mm to 500 mm; thepressure is 0.1 Pa to 2.0 Pa; direct current (DC) power supply is 0.25kW to 5.0 kW; the temperature is 20° C. to 100° C.; the atmosphere is arare gas atmosphere such as argon, an oxide atmosphere, or a mixedatmosphere of a rare gas such as argon and oxide.

Next, after the conductive layer 116 and the semiconductor layer 180with high conductivity are selectively etched to form the sourceelectrode 118, the drain electrode 120, the second wiring 122, and thesemiconductor layers 182, 184, and 186 with high conductivity, theisland-shape semiconductor layer 124 is formed so as to be partly incontact with the source electrode 118, the drain electrode 120, and thesemiconductor layers 182 and 184 with high conductivity in a regionoverlapped with the gate electrode 108 (see FIG. 21D).

The semiconductor layer 186 with high conductivity is provided over thesecond wiring 122 here; however, the present invention disclosed is notlimited thereto. The semiconductor layer with high conductivity may beformed so as to be in contact with at least the source electrode 118,the drain electrode 120, and the island-shape semiconductor layer 124.Further, before the island-shape semiconductor layer 124 is formed, asurface on which the island-shape semiconductor layer 124 is to beformed may be subjected to surface treatment. Embodiment 1 or the likecan be referred to for a specific example of surface treatment.

In this embodiment, the island-shape semiconductor layer 124 is formedby a sputtering method using an oxide semiconductor target containingIn, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1). The sputtering can be performedunder the following conditions, for example; the distance between thesubstrate 100 and the target is 30 mm to 500 mm; the pressure is 0.1 Pato 2.0 Pa; direct current (DC) power supply is 0.25 kW to 5.0 kW; thetemperature is 20° C. to 100° C.; the atmosphere is a rare gasatmosphere such as argon, an oxide atmosphere, or a mixed atmosphere ofa rare gas such as argon and oxide.

In this embodiment, film formation conditions of the semiconductor layer180 with high conductivity and the island-shape semiconductor layer 124are different. For example, a flow rate ratio of an oxygen gas to anargon gas in the film formation conditions of the semiconductor layer180 with high conductivity is smaller than that in the film formationconditions of the island-shape semiconductor layer 124. Morespecifically, the semiconductor layer with high conductivity is formedin a rare gas (such as argon or helium) atmosphere or an atmospherecontaining an oxygen gas at 10% or less and a rare gas at 90% or more.The semiconductor layer with normal conductivity is formed in an oxygenatmosphere or an atmosphere in which a flow rate of an oxygen gas is 1time or more that of a rare gas. In such a manner, two kinds ofsemiconductor layers having different conductivities can be formed.

In this embodiment, the case where the island-shape semiconductor layer124 is formed using an oxide semiconductor material is described;however, the present invention disclosed is not limited thereto. Theisland-shape semiconductor layer 124 may be formed using a semiconductormaterial such as silicon, germanium, silicon germanium, silicon carbide,gallium arsenide, or indium phosphide.

In addition, Embodiment 1 or the like may be referred to for the otherdetails.

Through the above steps, a transistor 190 in which the island-shapesemiconductor layer 124 is used as a channel formation region can beformed. Further, in a region where the second wiring 122 is overlappedwith the first wiring 110 (a region where the first wiring 110 and thesecond wiring 122 intersect with each other), a stacked-layer structureof the first wiring 110, the resist mask 112, the gate insulating layer114, the second wiring 122, and the semiconductor layer 186 with highconductivity can be formed. Thus, the capacitance value of the parasiticcapacitance can be reduced while suppressing increase in the number ofmanufacturing steps.

After that, a variety of electrodes and a wiring are formed, whereby asemiconductor device provided with the transistor 190 is completed.

As described in this embodiment, part of the resist masks formed using amulti-tone mask is provided between the first wiring and the secondwiring, whereby the capacitance value of the parasitic capacitance canbe reduced while suppressing increase in the number of manufacturingsteps.

Moreover, as described in this embodiment, the semiconductor layer withhigh conductivity is provided so as to be in contact with the sourceelectrode (or the gate electrode) and the island-shape semiconductorlayer, whereby electrical characteristics and reliability of atransistor can be improved. Thus, an excellent semiconductor device canbe provided.

Note that this embodiment can be implemented in combination with any ofthe other embodiments or example as appropriate.

Embodiment 12

In this embodiment, an example, which is different from the aboveembodiments, of a method for manufacturing a semiconductor device isdescribed with reference to drawings. Note that many parts of a step ofmanufacturing a semiconductor device in this embodiment are the same asthose in the other embodiments. Therefore, hereinafter, description forthe same parts as those of the above embodiments is omitted anddifferent parts from the above embodiments are described in detail.

First, the conductive layer 102 is formed over the substrate 100 and theresist masks 104 and 106 are selectively formed over the conductivelayer 102 (see FIG. 22A). The step is similar to the step in Embodiment1.

Next, after the conductive layer 102 is etched using the above resistmasks 104 and 106 to form the gate electrode 108 and the first wiring110, the resist masks 104 and 106 are made to recede to form the resistmask 112 over the first wiring 110, and the gate insulating layer 114 isformed so as to cover the resist mask 112, the gate electrode 108, andthe first wiring 110 which are formed (see FIG. 22B). The step is alsosimilar to the step in Embodiment 1; therefore, the detail is omitted.

Next, a semiconductor layer 181 with high conductivity and theconductive layer 116 are stacked in this order over the gate insulatinglayer 114 (see FIG. 22C).

There is no particular limitation on the semiconductor layer 181 withhigh conductivity as long as the semiconductor layer 181 with highconductivity has higher conductivity than an island-shape semiconductorlayer which is formed later. For example, in the case where theisland-shape semiconductor layer which is formed later is formed usingan oxide semiconductor material, a film formed of an oxide semiconductormaterial similar to that of the semiconductor layer with highconductivity can be formed under a different formation condition.Needless to say, the semiconductor layer 181 with high conductivity maybe formed using a different material from the island-shape semiconductorlayer which is formed later. In this embodiment, the case where thesemiconductor layer 181 with high conductivity and the island-shapesemiconductor layer which is formed later are formed using the samematerial is described.

In this embodiment, the semiconductor layer 181 with high conductivityis formed by a sputtering method using an oxide semiconductor targetcontaining In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1). The sputtering can beperformed under the following conditions, for example; the distancebetween the substrate 100 and the target is 30 mm to 500 mm; thepressure is 0.1 Pa to 2.0 Pa; direct current (DC) power supply is 0.25kW to 5.0 kW; the temperature is 20° C. to 100° C.; the atmosphere is arare gas atmosphere such as argon, an oxide atmosphere, or a mixedatmosphere of a rare gas such as argon and oxide.

The conductive layer 116 can be formed to have a single-layer structureof a molybdenum film or a titanium film. Alternatively, the conductivelayer 116 may be formed to have a stacked-layer structure and can have astacked-layer structure of an aluminum film and a titanium film, forexample. A three-layer structure in which a titanium film, an aluminumfilm, and a titanium film are stacked in this order may be employed. Athree-layer structure in which a molybdenum film, an aluminum film, anda molybdenum film are stacked in this order may be employed. Further, analuminum film containing neodymium (an Al—Nd film) may be used as thealuminum film used for these stacked-layer structures. Furtheralternatively, the conductive layer 116 may have a single-layerstructure of an aluminum film containing silicon. The detail of theconductive layer 102 or the like in Embodiment 1 can be referred to forthe detail of the conductive layer 116.

Next, after the conductive layer 116 and the semiconductor layer 181with high conductivity are selectively etched to form the sourceelectrode 118, the drain electrode 120, the second wiring 122, andsemiconductor layers 183, 185, and 187 with high conductivity, theisland-shape semiconductor layer 124 is formed so as to be partly incontact with the source electrode 118, the drain electrode 120, and thesemiconductor layers 183 and 185 with high conductivity in the regionoverlapped with the gate electrode 108 (see FIG. 22D).

Note that the semiconductor layer with high conductivity may be formedso as to be in contact with at least the source electrode 118, the drainelectrode 120, and the island-shape semiconductor layer 124. Further,before the island-shape semiconductor layer 124 is formed, a surface onwhich the island-shape semiconductor layer 124 is to be formed may besubjected to surface treatment. Embodiment 1 or the like can be referredto for a specific example of surface treatment.

In this embodiment, the island-shape semiconductor layer 124 is formed,for example, by a sputtering method using an oxide semiconductor targetcontaining In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1). The sputtering can beperformed under the following conditions, for example; the distancebetween the substrate 100 and the target is 30 mm to 500 mm; thepressure is 0.1 Pa to 2.0 Pa; direct current (DC) power supply is 0.25kW to 5.0 kW; the temperature is 20° C. to 100° C.; the atmosphere is arare gas atmosphere such as argon, an oxide atmosphere, or a mixedatmosphere of a rare gas such as argon and oxide.

In this embodiment, film formation conditions of the semiconductor layer181 with high conductivity and the island-shape semiconductor layer 124are different. For example, a flow rate ratio of an oxygen gas to anargon gas in the film formation conditions of the semiconductor layer181 with high conductivity is smaller than that in the film formationconditions of the island-shape semiconductor layer 124. Morespecifically, the semiconductor layer with high conductivity is formedin a rare gas (such as argon or helium) atmosphere or an atmospherecontaining an oxygen gas at 10% or less and a rare gas at 90% or more.The semiconductor layer with normal conductivity is formed in an oxygenatmosphere or an atmosphere in which a flow rate of an oxygen gas is 1time or more that of a rare gas. In such a manner, two kinds ofsemiconductor layers having different conductivities can be formed.

In this embodiment, the case where the island-shape semiconductor layer124 is formed using an oxide semiconductor material is described;however, the present invention disclosed is not limited thereto. Theisland-shape semiconductor layer 124 may be formed using a semiconductormaterial such as silicon, germanium, silicon germanium, silicon carbide,gallium arsenide, or indium phosphide.

In addition, Embodiment 1 or the like may be referred to for the otherdetails.

Through the above steps, a transistor 192 in which the island-shapesemiconductor layer 124 is used as a channel formation region can beformed. Further, in a region where the second wiring 122 is overlappedwith the first wiring 110 (a region where the first wiring 110 and thesecond wiring 122 intersect with each other), a stacked-layer structureof the first wiring 110, the resist mask 112, the gate insulating layer114, the semiconductor layer 187 with high conductivity, and the secondwiring 122 can be formed. Thus, the capacitance value of the parasiticcapacitance can be reduced while suppressing increase in the number ofmanufacturing steps.

After that, a variety of electrodes and a wiring are formed, whereby asemiconductor device provided with the transistor 192 is completed.

As described in this embodiment, part of the resist masks formed using amulti-tone mask is provided between the first wiring and the secondwiring, whereby the capacitance value of the parasitic capacitance canbe reduced while suppressing increase in the number of manufacturingsteps.

Moreover, as described in this embodiment, the semiconductor layer withhigh conductivity is provided so as to be in contact with the sourceelectrode (or the gate electrode) and the island-shape semiconductorlayer, whereby electrical characteristics and reliability of atransistor can be improved. Thus, an excellent semiconductor device canbe provided.

Note that this embodiment can be implemented in combination with any ofthe other embodiments or example as appropriate.

Embodiment 13

In this embodiment, an example, which is different from the aboveembodiments, of a method for manufacturing a semiconductor device isdescribed with reference to drawings. Note that many parts of a step ofmanufacturing a semiconductor device in this embodiment are the same asthose in the other embodiments. Therefore, hereinafter, description forthe same parts as those of the above embodiments is omitted anddifferent parts from the above embodiments are described in detail.

First, the conductive layer 102 is formed over the substrate 100 and theresist masks 104 and 106 are selectively formed over the conductivelayer 102 (see FIG. 23A). The step is similar to the step in Embodiment1.

Next, after the conductive layer 102 is etched using the above resistmasks 104 and 106 to form the gate electrode 108 and the first wiring110, the resist masks 104 and 106 are made to recede to form the resistmask 112 over the first wiring 110, and the gate insulating layer 114 isformed so as to cover the resist mask 112, the gate electrode 108, andthe first wiring 110 which are formed (see FIG. 23B). The step is alsosimilar to the step in Embodiment 1; therefore, the detail is omitted.

Next, the semiconductor layer 181 with high conductivity, the conductivelayer 116, and the semiconductor layer with high conductivity 180 arestacked in this order over the gate insulating layer 114 (see FIG. 23C).

There is no particular limitation on the semiconductor layers 180 and181 with high conductivity as long as the semiconductor layers 180 and181 with high conductivity have higher conductivity than an island-shapesemiconductor layer which is formed later. For example, in the casewhere the island-shape semiconductor layer which is formed later isformed using an oxide semiconductor material, a film formed of an oxidesemiconductor material similar to that of the semiconductor layer withhigh conductivity can be formed under a different formation condition.Needless to say, the semiconductor layers 180 and 181 with highconductivity may be formed using a different material from theisland-shape semiconductor layer which is formed later. Further, thesemiconductor layers 180 and 181 with high conductivity may be formedusing different materials from each other. In this embodiment, the casewhere the semiconductor layers 180 and 181 with high conductivity andthe island-shape semiconductor layer which is formed later are formedusing the same material is described.

In this embodiment, the semiconductor layers 180 and 181 with highconductivity are formed by a sputtering method using an oxidesemiconductor target containing In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1).The sputtering can be performed under the following conditions, forexample; the distance between the substrate 100 and the target is 30 mmto 500 mm; the pressure is 0.1 Pa to 2.0 Pa; direct current (DC) powersupply is 0.25 kW to 5.0 kW; the temperature is 20° C. to 100° C.; theatmosphere is a rare gas atmosphere such as argon, an oxide atmosphere,or a mixed atmosphere of a rare gas such as argon and oxide.

The conductive layer 116 can be formed to have a single-layer structureof a molybdenum film or a titanium film. Alternatively, the conductivelayer 116 may be formed to have a stacked-layer structure and can have astacked-layer structure of an aluminum film and a titanium film, forexample. A three-layer structure in which a titanium film, an aluminumfilm, and a titanium film are stacked in this order may be employed. Athree-layer structure in which a molybdenum film, an aluminum film, anda molybdenum film are stacked in this order may be employed. Further, analuminum film containing neodymium (an Al—Nd film) may be used as thealuminum film used for these stacked-layer structures. Furtheralternatively, the conductive layer 116 may have a single-layerstructure of an aluminum film containing silicon. The detail of theconductive layer 102 or the like in Embodiment 1 can be referred to forthe detail of the conductive layer 116.

Next, after the conductive layer 116 and the semiconductor layers 180and 181 with high conductivity are selectively etched to form the sourceelectrode 118, the drain electrode 120, the second wiring 122, and thesemiconductor layers 182, 183, 184, 185, 186, and 187 with highconductivity, the island-shape semiconductor layer 124 is formed so asto be partly in contact with the source electrode 118, the drainelectrode 120, and the semiconductor layers 182, 183, 184, and 185 withhigh conductivity in the region overlapped with the gate electrode 108(see FIG. 23D).

The semiconductor layer with high conductivity may be formed so as to bein contact with at least the source electrode 118, the drain electrode120, and the island-shape semiconductor layer 124. Further, before theisland-shape semiconductor layer 124 is formed, a surface on which theisland-shape semiconductor layer 124 is to be formed may be subjected tosurface treatment. Embodiment 1 or the like can be referred to for aspecific example of surface treatment.

In this embodiment, the island-shape semiconductor layer 124 is formedby a sputtering method using an oxide semiconductor target containingIn, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1). The sputtering can be performedunder the following conditions, for example; the distance between thesubstrate 100 and the target is 30 mm to 500 mm; the pressure is 0.1 Pato 2.0 Pa; direct current (DC) power supply is 0.25 kW to 5.0 kW; thetemperature is 20° C. to 100° C.; the atmosphere is a rare gasatmosphere such as argon, an oxide atmosphere, or a mixed atmosphere ofa rare gas such as argon and oxide.

In this embodiment, film formation conditions of the semiconductorlayers 180 and 181 with high conductivity and the island-shapesemiconductor layer 124 are different. For example, a flow rate ratio ofan oxygen gas to an argon gas in the film formation conditions of thesemiconductor layers 180 and 181 with high conductivity is smaller thanthat in the film formation conditions of the island-shape semiconductorlayer 124. More specifically, the semiconductor layer with highconductivity is formed in a rare gas (such as argon or helium)atmosphere or an atmosphere containing an oxygen gas at 10% or less anda rare gas at 90% or more. The semiconductor layer with normalconductivity is formed in an oxygen atmosphere or an atmosphere in whicha flow rate of an oxygen gas is 1 time or more that of a rare gas. Insuch a manner, two kinds of semiconductor layers having differentconductivities can be formed.

In this embodiment, the case where the island-shape semiconductor layer124 is formed using an oxide semiconductor material is described;however, the present invention disclosed is not limited thereto. Theisland-shape semiconductor layer 124 may be formed using a semiconductormaterial such as silicon, germanium, silicon germanium, silicon carbide,gallium arsenide, or indium phosphide.

In addition, Embodiment 1 or the like may be referred to for the otherdetails.

Through the above steps, a transistor 194 in which the island-shapesemiconductor layer 124 is used as a channel formation region can beformed. Further, in a region where the second wiring 122 is overlappedwith the first wiring 110 (a region where the first wiring 110 and thesecond wiring 122 intersect with each other), a stacked-layer structureof the first wiring 110, the resist mask 112, the gate insulating layer114, the semiconductor layer 187 with high conductivity, the secondwiring 122, and the semiconductor layer 186 with high conductivity canbe formed. Thus, the capacitance value of the parasitic capacitance canbe reduced while suppressing increase in the number of manufacturingsteps.

After that, a variety of electrodes and a wiring are formed, whereby asemiconductor device provided with the transistor 194 is completed.

As described in this embodiment, part of the resist masks formed using amulti-tone mask is provided between the first wiring and the secondwiring, whereby the capacitance value of the parasitic capacitance canbe reduced while suppressing increase in the number of manufacturingsteps.

Moreover, as described in this embodiment, the semiconductor layer withhigh conductivity is provided so as to be in contact with the sourceelectrode (or the gate electrode) and the island-shape semiconductorlayer, whereby electrical characteristics and reliability of atransistor can be improved. Thus, an excellent semiconductor device canbe provided.

Note that this embodiment can be implemented in combination with any ofthe other embodiments or example as appropriate.

Example 1

In this example, in order to confirm an effect of the present inventiondisclosed, current-voltage characteristics and mobility characteristicsof a transistor were examined. Description is hereinafter made withreference to drawings.

Examination of this example was performed using a transistor(hereinafter, a transistor B) according to Embodiment 12 (see FIG. 24B).For comparison, a similar examination was performed on a transistor(hereinafter, a transistor A) in which a semiconductor layer with highconductivity which is under a source electrode (or a drain electrode) isnot provided (see FIG. 24A).

A method for manufacturing transistors followed those of Embodiment 12.Here, the only difference in a manufacturing step between thetransistors A and B is whether there is a step of forming thesemiconductor layer with high conductivity which is under the sourceelectrode (or the drain electrode) or not. Note that titanium was usedfor the source electrode (or the drain electrode) and an oxidesemiconductor material containing indium, gallium, and zinc was used forthe semiconductor layer with high conductivity and an island-shapesemiconductor layer. In addition, before the island-shape semiconductorlayer is formed, reverse sputtering is performed as surface treatment.The channel length of the transistors was 20 μm and the channel widththereof was 20 nm. The thickness of the semiconductor layers with highconductivity was 5 nm.

FIG. 25A shows current-voltage characteristics and mobilitycharacteristics of the transistor A and FIG. 25B shows current-voltagecharacteristics and mobility characteristics of the transistor B. Thehorizontal axis indicates gate voltage (Vg) and the vertical axisindicates a current value (Id) or field effect mobility (μFE). Here,source-drain voltage was 10 V. In FIG. 25A, there were large variationsin current-voltage characteristics. On the other hand, in FIG. 25B,there were extremely small variations in current-voltagecharacteristics.

The details of the above phenomenon are unclear; however, improvement ofelectrical connection between the island-shape semiconductor layer andthe source electrode (or the drain electrode) due to the semiconductorlayer with high conductivity, or the like is considered as a cause ofthat.

In such a manner, the semiconductor layer with high conductivity isprovided between the source electrode (or the drain electrode) and theisland-shape semiconductor layer, whereby a semiconductor device withexcellent electrical characteristics can be provided. This example canbe implemented in combination with any of the other embodiments asappropriate.

This application is based on Japanese Patent Application serial No.2008-330258 filed with Japan Patent Office on Dec. 25, 2008, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a first wiring overa substrate; a gate insulating layer over the first wiring; an oxidesemiconductor layer over the gate insulating layer; a second wiringelectrically connected to the oxide semiconductor layer, wherein thenumber of insulating layers between the first wiring and the oxidesemiconductor layer is different from the number of insulating layersbetween the first wiring and the second wiring.
 3. The semiconductordevice according to claim 2, wherein a portion of the first wiring is agate electrode.
 4. The semiconductor device according to claim 2,wherein a portion of the second wiring is a source electrode.
 5. Thesemiconductor device according to claim 2, further comprising a resistmask between the first wiring and the second wiring.
 6. Thesemiconductor device according to claim 2, further comprising atransparent conductive layer over and in contact with the second wiring.7. The semiconductor device according to claim 2, wherein thesemiconductor device is one selected from the group consisting of ane-book reader, a television set, a digital photo frame, an amusementmachine, and a cellular phone.
 8. A semiconductor device comprising: afirst wiring over a substrate; a gate insulating layer over the firstwiring; an oxide semiconductor layer over the gate insulating layer; asecond wiring electrically connected to the oxide semiconductor layer,wherein a total thickness of insulating layers between the first wiringand the oxide semiconductor layer is different from a total thickness ofinsulating layers between the first wiring and the second wiring.
 9. Thesemiconductor device according to claim 8, wherein a portion of thefirst wiring is a gate electrode.
 10. The semiconductor device accordingto claim 8, wherein a portion of the second wiring is a sourceelectrode.
 11. The semiconductor device according to claim 8, furthercomprising a resist mask between the first wiring and the second wiring.12. The semiconductor device according to claim 8, further comprising atransparent conductive layer over and in contact with the second wiring.13. The semiconductor device according to claim 8, wherein thesemiconductor device is one selected from the group consisting of ane-book reader, a television set, a digital photo frame, an amusementmachine, and a cellular phone.